MIPS: remove CONFIG_SYS_MHZ

Resolve all uses of CONFIG_SYS_MHZ with the currently defined value.
Remove code which depends on CONFIG_SYS_MHZ but where no board configs
actually use that code.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>