ARM: OMAP2+: Fix build issues with missing include of linux/bug.h

Otherwise we can get the following if bug.h is not included from kernel.h:

arch/arm/mach-omap2/powerdomain-common.c:
In function 'omap2_pwrdm_get_mem_bank_onstate_mask':
arch/arm/mach-omap2/powerdomain-common.c:64:3: error:
implicit declaration of function 'WARN_ON' [-Werror=implicit-function-declaration]
cc1: some warnings being treated as errors

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