ARM: OMAP2+: Fix build for dra7xx without omap4 and 5

Otherwise we can get errors like:

arch/arm/mach-omap2/prm44xx.c:274: error: redefinition of ‘omap44xx_prm_reconfigure_io_chain’
arch/arm/mach-omap2/built-in.o: In function `default_finish_suspend':
arch/arm/mach-omap2/omap-mpuss-lowpower.c:95: undefined reference to `omap_do_wfi'

Signed-off-by: Tony Lindgren <tony@atomide.com>