OMAP: hwmod: fix build break on non-OMAP4 multi-OMAP2 builds

OMAP: hwmod: fix build break on non-OMAP4 multi-OMAP2 builds

Builds for multi-OMAP2 (e.g., OMAP2420 with OMAP2430) with
CONFIG_ARCH_OMAP4=n fail with the following errors:

arch/arm/mach-omap2/built-in.o: In function `_enable_module':
arch/arm/mach-omap2/omap_hwmod.c:701: undefined reference to `omap4_cminst_module_enable'
arch/arm/mach-omap2/built-in.o: In function `_disable_module':
arch/arm/mach-omap2/omap_hwmod.c:726: undefined reference to `omap4_cminst_module_disable'
arch/arm/mach-omap2/built-in.o: In function `_wait_target_disable':
arch/arm/mach-omap2/omap_hwmod.c:1179: undefined reference to `omap4_cminst_wait_module_idle'

This is probably due to the preprocessor directives in
arch/arm/plat-omap/include/plat/cpu.h that convert some cpu_is_omap*()
expressions from preprocessor directives into something that is only
resolvable during runtime, if multiple OMAP2 build targets are
selected.

Thanks to Tony Lindgren <tony@atomide.com> for reporting.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>