configs/beaglev_defconfig: 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>