ARM: OMAP4: Don't compile cm2xxx_3xxx.c for OMAP4 only builds.

Since OMAP4 code base now makes use of OMAP4 specific PRCM functions,
cm2xxx_3xxx.c need not be compiled for OMAP4 only builds.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Paul Walmsley <paul@pwsan.com>