OMAP2: cpu_is_omap2*: fix compile-time removal of unused code

Currently if omap2420 is defined but not omap2430, cpu_is_omap2430()
is still defined as a macro, instead of #define'd to zero.  This
results in conditional cpu_is_omap2430() code still being compiled,
and leads to possible compile/link errors.  In particular for hwmod
init.

To fix, add extra #ifdefs to CPU check macros to ensure that the
is_omap* macros are zero for each OMAP2 if they are not configured
into the kernel.

Tested-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>