configs/hifive_unleashed: Enable BR2_RISCV_ISA_CUSTOM_RVA explicitly

BR2_RISCV_ISA_CUSTOM_RVA is currently not explicitly listed as it is
selected by BR2_riscv_custom. This looks confusing. Let's enable it.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

configs/hifive_unleashed_defconfig

@@ -2,6 +2,7 @@
 BR2_riscv=y
 BR2_riscv_custom=y
 BR2_RISCV_ISA_CUSTOM_RVM=y
+BR2_RISCV_ISA_CUSTOM_RVA=y
 BR2_RISCV_ISA_CUSTOM_RVF=y
 BR2_RISCV_ISA_CUSTOM_RVD=y
 BR2_RISCV_ISA_CUSTOM_RVC=y