ARM: OMAP2+: PRM: fix compile for OMAP4-only build

ARM: OMAP2+: PRM: fix compile for OMAP4-only build

For OMAP4 only builds, the omap2_prm_* functions have dummy wrappers
to detect incorrect usage.  However, several unrelated omap3 PRM
functions have made it inside the #else clause of the #ifdef wrapping
the omap2_prm stubs, causing them to disappear on OMAP4-only builds.

This was unnoticed until the IO chain support was added and introduced
a new function in this section which is referenced by omap_hwmod.c:

arch/arm/mach-omap2/omap_hwmod.c: In function '_reconfigure_io_chain':
arch/arm/mach-omap2/omap_hwmod.c:1665:3: error: implicit declaration of function 'omap3xxx_prm_reconfigure_io_chain' [-Werror=implicit-function-declaration]

Fix by using the #ifdef to only wrap the omap2_prm functions that
need stubs on OMAP4-only builds.

Cc: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>
[paul@pwsan.com: fixed checkpatch warnings for patch description]
Signed-off-by: Paul Walmsley <paul@pwsan.com>