ARM: OMAP2+: Fix GPMC remap for devices using an offset

At least the smc91x driver expects the device to be at 0x300
offset from bus base address. This does not work currently
for GPMC when booted in device tree mode as it attempts to
remap the the allocated GPMC partition to the address
configured by the device tree plus the device offset.

Note that this works just fine when booted with legacy mode.

Let's fix the issue by just ignoring any device specific
offset while remapping. And let's make sure the remap
address confirms to the GPMC 16MB minimum granularity
as listed in the TRM for GPMC_CONFIG7 BASEADDRESS bits.

Otherwise we can get something like this:

omap-gpmc 6e000000.gpmc: cannot remap GPMC CS 1 to 0x01000300

Cc: Pekon Gupta <pekon@ti.com>
Reviewed-by: Javier Martinez Canillas <javier@dowhile0.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>