arch/arm: add Armv9-A and the Neoverse N2 core

This commit adds the Neoverse N2 core, which is an implementation of
the armv9.0a ISA. See: [1] [2].

This CPU support was added in GCC 11. See [3].

The Neoverse N2 CPU is included in the Arm sbsa-ref platform in Qemu.
The Armv9.0-A Buildroot support will be needed to update those SBSA
related components. See the note in commit log [4].

This commit introduces the BR2_ARM_CPU_ARMV9A Kconfig symbol, which just
selects BR2_ARM_CPU_ARMV8A. This is because Armv9.0-A is a superset of
Armv8.5-A. This symbol can help package recipes to detect this new base
architecture.

Neoverse-N2 support has been added in QEmu in commit [5], first
included in v8.2.0.

This CPU supports Aarch32 only in EL0 (user-space). This means it's
technically possible to compile Aarch32 code targeting it. GCC has
the support to do so. Since Buildroot recompiles a full system (ATF,
Kernel, user-space) this support has limited value. This is why this
CPU is limited to 64bit builds only.

[1] https://developer.arm.com/Processors/Neoverse%20N2
[2] https://developer.arm.com/documentation/102099/0003/The-Neoverse-N2--core/Neoverse-N2--core-features
[3] https://gcc.gnu.org/gcc-11/changes.html
[4] https://gitlab.com/buildroot.org/buildroot/-/commit/c58a2a9687c045379c0f768c8c6c1c1a41842a85
[5] https://gitlab.com/qemu-project/qemu/-/commit/dfff1000fef24f6686e0be5e6472613985a363dc

Signed-off-by: Julien Olivain <ju.o@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

arch/Config.in.arm

@@ -95,6 +95,14 @@ config BR2_ARM_CPU_ARMV8A
 	bool
 	select BR2_USE_MMU
 
+# Armv9.0-A base specification is a super-set of Armv8.5-A. It is
+# fully compatible with Armv8. This symbol is defined to allow checks
+# for doing Armv9 optimization. Armv8 is also selected for
+# compatibility.
+config BR2_ARM_CPU_ARMV9A
+	bool
+	select BR2_ARM_CPU_ARMV8A
+
 choice
 	prompt "Target Architecture Variant"
 	default BR2_cortex_a53 if BR2_ARCH_IS_64
@@ -496,6 +504,14 @@ config BR2_saphira
 	select BR2_ARM_CPU_HAS_FP_ARMV8
 	select BR2_ARM_CPU_ARMV8A
 	select BR2_ARCH_NEEDS_GCC_AT_LEAST_8
+
+comment "armv9.0a cores"
+config BR2_neoverse_n2
+	bool "neoverse-N2 (aka perseus)"
+	depends on BR2_ARCH_IS_64
+	select BR2_ARM_CPU_HAS_FP_ARMV8
+	select BR2_ARM_CPU_ARMV9A
+	select BR2_ARCH_NEEDS_GCC_AT_LEAST_11
 endchoice
 
 config BR2_ARM_ENABLE_NEON
@@ -889,6 +905,8 @@ config BR2_GCC_TARGET_CPU
 	default "tsv110"	if BR2_tsv110
 	# armv8.4a
 	default "saphira"	if BR2_saphira
+	# armv9.0a
+	default "neoverse-n2"	if BR2_neoverse_n2
 
 config BR2_GCC_TARGET_ABI
 	default "aapcs-linux"	if BR2_arm || BR2_armeb