omap2plus_defocnfig build breaks when customised with only ARCH_OMAP4
selected. This is because common files make references to the functions
which are defined only for omap2xxx and omap3xxx.

 LD      .tmp_vmlinux1
arch/arm/mach-omap2/built-in.o: In function `pm_dbg_regset_store':
arch/arm/mach-omap2/pm-debug.c:335: undefined reference to `omap2_prm_read_mod_reg'
arch/arm/mach-omap2/built-in.o: In function `omap2_pm_dump':
arch/arm/mach-omap2/pm-debug.c:121: undefined reference to `omap2_prm_read_mod_reg'
arch/arm/mach-omap2/pm-debug.c:123: undefined reference to `omap2_prm_read_mod_reg'
arch/arm/mach-omap2/pm-debug.c:124: undefined reference to `omap2_prm_read_mod_reg'
arch/arm/mach-omap2/pm-debug.c:125: undefined reference to `omap2_prm_read_mod_reg'
arch/arm/mach-omap2/built-in.o: In function `omap_prcm_arch_reset':
arch/arm/mach-omap2/prcm.c:106: undefined reference to `omap2_prm_set_mod_reg_bits'
arch/arm/mach-omap2/prcm.c:108: undefined reference to `omap2_prm_read_mod_reg'
arch/arm/mach-omap2/built-in.o: In function `omap_prcm_get_reset_sources':
arch/arm/mach-omap2/prcm.c:53: undefined reference to `omap2_prm_read_mod_reg'
arch/arm/mach-omap2/built-in.o: In function `clkdm_clear_all_wkdeps':
arch/arm/mach-omap2/clockdomain.c:545: undefined reference to `omap2_prm_clear_mod_reg_bits'
arch/arm/mach-omap2/built-in.o: In function `clkdm_del_wkdep':
arch/arm/mach-omap2/clockdomain.c:475: undefined reference to `omap2_prm_clear_mod_reg_bits'
arch/arm/mach-omap2/built-in.o: In function `clkdm_read_wkdep':
arch/arm/mach-omap2/clockdomain.c:511: undefined reference to `omap2_prm_read_mod_bits_shift'
arch/arm/mach-omap2/built-in.o: In function `clkdm_add_wkdep':
arch/arm/mach-omap2/clockdomain.c:440: undefined reference to `omap2_prm_set_mod_reg_bits'
make: *** [.tmp_vmlinux1] Error 1

This patch adds stubs for these functions so that build continues to work.
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Acked-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>