ARM: OMAP: Clear GPMC bits when applying new setting.

When setting the GPMC device type, make sure any previous
bits are cleared down, before applying the new setting.

For OMAP4+ devices MUXADDDATA is a 2-bit field (bits 9:8)
where as for OMAP2/3 devices it was only a one bit field
(bit 9). For OMAP2/3 devices bit 8 is reserved and the
OMAP documentation says to write a 0 to this bit. So
clearing bit 8 on OMAP2/3 devices should not be a problem.
Hence update the code to handle both bits 8 and 9 for all
devices.

Signed-off-by: Mark Jackson <mpfj@newflow.co.uk>
[jon-hunter@ti.com: updated changelog]
Signed-off-by: Jon Hunter <jon-hunter@ti.com>