omap_gpmc: Accept a zero mask field on omap3630

OMAP3630 adds an extra bit of address masking, so a mask of
0xb1111 is valid. Unfortunately the GPMC_REVISION is the same as
on the OMAP3430 which only has three bits of address masking, so
we have to derive this feature directly from the OMAP revision
rather than from the GPMC revision.

Signed-off-by: Juha Riihimäki <juha.riihimaki@nokia.com>
[Riku Voipio: Fixes and restructuring patchset]
Signed-off-by: Riku Voipio <riku.voipio@iki.fi>
[Peter Maydell: More fixes and cleanups for upstream submission]
Signed-off-by:  Peter Maydell <peter.maydell@linaro.org>
