If we don't have ARCH_OMAP2, 3 or 4 selected randconfig will always
fail with multiple errors as the CPU and MACHINE are not set.

Fix this by changing arch/arm/Makefile to build mach-omap2 based on
ARCH_OMAP2PLUS. And let's introduce SOC_OMAP and SOC_OMAP_NOOP that
allow randconfig to generate buildable .config files.

Note that we can also remove few uncecssary ARCH_OMAP2PLUS lines
as they are all within if ARCH_OMAP2PLUS block.

Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Tony Lindgren <tony@atomide.com>