ARM: OMAP: Fix errors and warnings when building for one board

When customizing omap2plus_defconfig to build for only
one board (omap3evm), I came across these warnings and
errors (filenames truncated):

arch/arm/mach-omap2/board-generic.c:76:20: warning: 'omap4_init' defined but not used
arch/arm/mach-omap2/built-in.o: In function `omap2420_init_early':
arch/arm/mach-omap2/io.c:364: undefined reference to `omap2_set_globals_242x'
arch/arm/mach-omap2/io.c:366: undefined reference to `omap2xxx_voltagedomains_init'
arch/arm/mach-omap2/io.c:367: undefined reference to `omap242x_powerdomains_init'
arch/arm/mach-omap2/io.c:368: undefined reference to `omap242x_clockdomains_init'
arch/arm/mach-omap2/io.c:369: undefined reference to `omap2420_hwmod_init'
arch/arm/mach-omap2/built-in.o: In function `omap2430_init_early':
arch/arm/mach-omap2/io.c:376: undefined reference to `omap2_set_globals_243x'
arch/arm/mach-omap2/io.c:378: undefined reference to `omap2xxx_voltagedomains_init'
arch/arm/mach-omap2/io.c:379: undefined reference to `omap243x_powerdomains_init'
arch/arm/mach-omap2/io.c:380: undefined reference to `omap243x_clockdomains_init'
arch/arm/mach-omap2/io.c:381: undefined reference to `omap2430_hwmod_init'
arch/arm/mach-omap2/built-in.o: In function `omap4430_init_early':
arch/arm/mach-omap2/io.c:436: undefined reference to `omap2_set_globals_443x'
arch/arm/mach-omap2/io.c:438: undefined reference to `omap44xx_voltagedomains_init'
arch/arm/mach-omap2/io.c:439: undefined reference to `omap44xx_powerdomains_init'
arch/arm/mach-omap2/io.c:440: undefined reference to `omap44xx_clockdomains_init'
arch/arm/mach-omap2/io.c:441: undefined reference to `omap44xx_hwmod_init'

This patch fixes them.

Signed-off-by: Sanjeev Premi <premi@ti.com>
Acked-by: Thomas Weber <weber@corscience.de>
[tony@atomide.com: updated to fix warnings for board-generic.c]
Signed-off-by: Tony Lindgren <tony@atomide.com>