riscv: ae350: Use #if defined instead of CONFIG_IS_ENABLED

According to ./include/linux/kconfig.h,
CONFIG_IS_ENABLED(OF_BOARD) expands to 0
when CONFIG_SPL_BUILD is defined because
there is no CONFIG_SPL_OF_BOARD.

Use #if defined instead.

Fixes: 2e8d2f88 ("riscv: Remove OF_PRIOR_STAGE from RISC-V boards")

Signed-off-by: Leo Yu-Chi Liang <ycliang@andestech.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>