boot: Drop CMD_MTDPARTS condition for FDT_FIXUP_PARTITIONS

This is not needed, so drop it. Also use a capital 'O' for the option,
while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>