ARM: OMAP2+: prcm: add omap_test_timeout to prcm-common.h

Done in preparation to move cm/prm to drivers. These will still use
omap_test_timeout, but will not have access to common.h header under
mach-omap2 anymore.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>