powerpc: Fix incorrect SYS_IMMR migration values

When migrating SYS_IMMR, I didn't allow for boards to provide
non-default values here.  This lead to an incorrect migration on the
platforms where CONFIG_SYS_IMMR is set to CONFIG_SYS_CCSRBAR and
CONFIG_SYS_CSSRBAR is NOT the same as CONFIG_SYS_CCSRBAR_DEFAULT.  Add
text to the prompt so that non-default values can be used and re-migrate
the platforms that have CONFIG_SYS_IMMR=CONFIG_SYS_CSSRBAR where
CONFIG_SYS_CSSRBAR != CONFIG_SYS_CCSRBAR_DEFAULT.

Fixes: be7dbb60 ("Convert CONFIG_SYS_IMMR to Kconfig")
Reported-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Tested-by: Pali Rohár <pali@kernel.org>