OMAP2+: Kconfig: disallow builds for boards that don't use the currently-selected SoC

Currently, if, for example, CONFIG_ARCH_OMAP2420 is not selected, OMAP2420
board files can still be included in the build.  This results in link errors:

arch/arm/mach-omap2/built-in.o: In function `omap_generic_map_io':
.../arch/arm/mach-omap2/board-generic.c:51: undefined reference to `omap2_set_globals_242x'
arch/arm/mach-omap2/built-in.o: In function `omap_h4_init':
.../arch/arm/mach-omap2/board-h4.c:330: undefined reference to `omap2420_mux_init'
arch/arm/mach-omap2/built-in.o: In function `omap_h4_map_io':
.../arch/arm/mach-omap2/board-h4.c:373: undefined reference to `omap2_set_globals_242x'
arch/arm/mach-omap2/built-in.o: In function `omap_apollon_init':
.../arch/arm/mach-omap2/board-apollon.c:325: undefined reference to `omap2420_mux_init'
arch/arm/mach-omap2/built-in.o: In function `omap_apollon_map_io':
.../arch/arm/mach-omap2/board-apollon.c:353: undefined reference to `omap2_set_globals_242x'
make: *** [.tmp_vmlinux1] Error 1

Fix this by making the boards depend on the Kconfig option for the
specific SoC that they use.

Also, while here, fix the mach-omap2/board-generic.c file to remove the
dependency on OMAP2420.

Charulatha Varadarajan <charu@ti.com> caught a typo - thanks Charu.

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