arch/Config.in.riscv: lp64f ABI is only supported if MMU is enabled

Even though that seems weird, the LP64F ABI is only supported when MMU
support is enabled. Indeed, as per commit
9a51381cedc16e6d70cb85e1144f6e0fa89af69a ("package/uclibc: prevent
config with unsupported RISC-V float ABI"), uClibc does not support
LP64F. But uClibc is the only C library that support RISC-V 64-bit
noMMU.

So the selection of LP64F and !MMU is impossible. Right now this
selection causes a build failure as no C library is enabled.

This commit fixes this by ensuring we cannot use LP64F when MMU
support is not available.

Fixes:

  http://autobuild.buildroot.net/results/6320dcc655f1871eb8bf6a9689bddde7447385f5/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit f7d60ee57d7d5155d4b3ac5f5ca12b7445faf95c)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

arch/Config.in.riscv

@@ -108,6 +108,7 @@ config BR2_RISCV_ABI_LP64
 config BR2_RISCV_ABI_LP64F
 	bool "lp64f"
 	depends on BR2_ARCH_IS_64 && BR2_RISCV_ISA_RVF
+	depends on BR2_USE_MMU
 
 config BR2_RISCV_ABI_LP64D
 	bool "lp64d"