ARM: OMAP2+: io: fix compilation breakage on 2420-only configs

Commit 7b250aff ("ARM: OMAP: Avoid
cpu_is_omapxxxx usage until map_io is done") breaks the build on a
2420-only config on v3.3-rc1:

arch/arm/mach-omap2/built-in.o: In function `omap2430_init_early':
arch/arm/mach-omap2/io.c:406: undefined reference to `omap2_set_globals_243x'
arch/arm/mach-omap2/io.c:410: undefined reference to `omap243x_clockdomains_init'
arch/arm/mach-omap2/io.c:411: undefined reference to `omap2430_hwmod_init'

Fix by only compiling omap2420_init_early() when CONFIG_SOC_OMAP2420
is selected, and only compiling omap2430_init_early() when
CONFIG_SOC_OMAP2430 is selected.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Tony Lindgren <tony@atomide.com>