ARM: mach-shmobile: sh7372 Mackerel NOR Flash USB boot fix

Always use CS0 shadow area for NOR flash instead of regular CS0
memory area on Mackerel.

When booting from CS0 NOR Flash the regular CS0 memory area is
available, but when booting via USB the MASK ROM gets mapped to
0x0 which gets in the way for the NOR Flash. Always using CS0
shadow area works well for both NOR Flash boot and USB boot.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>