arch/Config.in.riscv: Remove BR2_RISCV_ISA_CUSTOM_RVA from BR2_riscv_custom
BR2_riscv_custom should only contain the basic I extension. The A
extension should be optional and selected by board configs.
Signed-off-by: Bin Meng <bmeng@tinylab.org>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>