[ARM] Move rest of MMU setup code from mm-armv.c to mmu.c

If we're going to have mmu.c for code which is specific to the MMU
machines, we might as well move the other MMU initialisation
specific code from mm-armv.c into this new file.  This also allows
us to make some functions static.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>