Blackfin: push BF5Xx configs down into mach-specific Kconfigs

While we're moving the BF54x code, have the BF54xM variants select the
normal BF54x values so that the rest of the Kconfig tree doesn't need to
check the BF54xM variant everytime it wants to check the BF54x.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>