OMAP: Fix linking error in twl-common.c for OMAP2/3/4 only builds

Commit b22f954 (OMAP4: Move common twl6030 configuration to twl-common)
caused compile failures for code for OMAP arch which is not selected by
the config.

Fixes issues like:
With CONFIG_ARCH_OMAP3=y and CONFIG_ARCH_OMAP4=n, I'm getting this:

arch/arm/mach-omap2/built-in.o:(.data+0xf99c): undefined reference to `omap4430_phy_init'
arch/arm/mach-omap2/built-in.o:(.data+0xf9a0): undefined reference to `omap4430_phy_exit'
arch/arm/mach-omap2/built-in.o:(.data+0xf9a4): undefined reference to `omap4430_phy_power'
arch/arm/mach-omap2/built-in.o:(.data+0xf9a8): undefined reference to `omap4430_phy_set_clk'
arch/arm/mach-omap2/built-in.o:(.data+0xf9ac): undefined reference to `omap4430_phy_suspend'

Fix the problem by moving the code to ifdef sections for omap3 and omap4.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
[tony@atomide.com: updated comments]
Signed-off-by: Tony Lindgren <tony@atomide.com>