Convert CONFIG_SYS_IMMR to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_IMMR

We do this by consolidating the SYS_IMMR options we have and providing
defaults.

We also, in the few places where M68K was also sharing code with these
platforms, define it within the file to CONFIG_SYS_MBAR to match usage.
This should be cleaned up longer term.

Signed-off-by: Tom Rini <trini@konsulko.com>