MIPS: bmips: switch to CONFIG_OF_SEPARATE

Fix the Kconfig warning to not use CONFIG_OF_EMBED in defconfigs.

Based on https://patchwork.ozlabs.org/patch/1019791/



Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>