cpufreq: OMAP: fix build errors: depends on ARCH_OMAP2PLUS

The OMAP driver needs a 'depends on ARCH_OMAP2PLUS' since it only
builds for OMAP2+ platforms.

This 'depends on' was in the original patch from Russell King, but was
erroneously removed by me when making this option user-selectable in
commit b09db45c (cpufreq: OMAP driver depends CPUfreq tables.) This
patch remedies that.

Apologies to Russell King for breaking his originally working patch.

Also, thanks to Grazvydas Ignotas for reporting the same problem.

Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>