OMAP: split plat-omap/common.c

Split plat-omap/common.c into three pieces:

1. the 32KiHz sync timer and clocksource code, which now lives in
   plat-omap/counter_32k.c;

2. the OMAP2+ common code, which has been moved to mach-omap2/common.c;

3. and the remainder of the OMAP-wide common code, which includes the
   deprecated ATAGs code and a deprecated video RAM reservation function.

The primary motivation for doing this is to move the OMAP2+-specific parts
into an OMAP2+-specific file, so that build breakage related to the
System Control Module code can be resolved.

Benoît Cousson <b-cousson@ti.com> suggested a new filename and found
some bugs in the counter_32k.c comments - thanks Benoît.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Benoît Cousson <b-cousson@ti.com>