ARM: OMAP2+: Fix build for omap4 only builds with missing include of linux/bug.h

Found few more with randconfig generated .configs:

In file included from arch/arm/mach-omap2/prm-regbits-34xx.h:17,
                 from arch/arm/mach-omap2/vc.c:18:
arch/arm/mach-omap2/prm2xxx_3xxx.h: In function ‘omap2_prm_read_mod_reg’:
arch/arm/mach-omap2/prm2xxx_3xxx.h:239: error: implicit declaration of function ‘WARN’

In file included from arch/arm/mach-omap2/powerdomain44xx.c:22:
arch/arm/mach-omap2/prm2xxx_3xxx.h: In function ‘omap2_prm_read_mod_reg’:
arch/arm/mach-omap2/prm2xxx_3xxx.h:239: error: implicit declaration of function ‘WARN’

This is because omap2_prm functions are currently just stubs for
omap4 only builds.

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