ARM: OMAP: Remove plat-omap/common.h

Most of the prototypes in plat-omap/common.h are not
common to omap1 and omap2+, they are local to omap2+
and should not be in plat-omap/common.h.

The only shared function prototype in this file is
omap_init_clocksource_32k(), let's put that into
counter-32k.h.

Note that the new plat/counter-32k.h must not be
included from drivers, that will break omap2+ build
for CONFIG_MULTIPLATFORM.

Signed-off-by: Tony Lindgren <tony@atomide.com>