ARM: omap: move OMAP specific code under arch/arm/

This patch moves OMAP specific directory entries
from the toplevel Makefile and spl/Makefile
to arch/arm/cpu/armv7/Makefile using Kbuild descending feature.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Tom Rini <trini@ti.com>