ARM: mach-shmobile: Correctly check for CONFIG_MACH_MACKEREL

I made a bit of a thinko when adding Mackerel to the boards
that support zboot using MMCIF.

Reported-by: Magnus Damm <magnus.damm@gmail.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>