ARM: OMAP2+: remove redundant multiplatform checks

The directory arch/arm/mach-omap2 is only selected for compilation if
CONFIG_ARCH_OMAP2PLUS is selected. CONFIG_ARCH_OMAP2PLUS itself is a
silent option and all machines selecting this option are multiplatform
devices. As a consequence checks for CONFIG_ARCH_MULTIPLATFORM as well
as CONFIG_ARCH_OMAP2PLUS within that directory are superfluous and can
be removed.

Signed-off-by: Jonas Rabenstein <jonas.rabenstein@studium.uni-erlangen.de>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>