riscv: Enable CONFIG_OF_BOARD_FIXUP by default for OF_SEPARATE

Starting from OpenSBI v0.7, the SBI firmware inserts/fixes up the
reserved memory node for PMP protected memory regions. All RISC-V
boards need to copy the reserved memory node from the device tree
provided by the firmware to the device tree used by U-Boot.

Turn on CONFIG_OF_BOARD_FIXUP by default for OF_SEPARATE.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Rick Chen <rick@andestech.com>