arm: remove ability to select CONFIG_MCA

It isn't clear to me why this ever existed, as I've never heard
of an ARM board with an MCA bus.  Regardless, the MCA bus support
is going away, so remove the ability to select it from ARM.

Cc: Russell King <linux@arm.linux.org.uk>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>