riscv: Only enable OF_BOARD_FIXUP for S-Mode

It is unsafe to enable OF_BOARD_FIXUP only based on OF_SEPARATE.
OF_SEPARATE may indicate that the user wishes U-Boot to use a different
device tree than one obtained via OF_PRIOR_STAGE. However, OF_SEPARATE may
also indicate that the device tree which would be obtained via
OF_PRIOR_STAGE is invalid, nonexistant, or otherwise unusable. In this
latter case, enabling OF_BOARD_FIXUP will result in corruption of the
device tree. To remedy this, only enable OF_BOARD_FIXUP if U-Boot is
configured for S-Mode.

Fixes: 1c17e555
Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Leo Liang <ycliang@andestech.com>