ARM: All OMAP2PLUS machines use omap2 directory so just add one entry

All OMAP2PLUS arch based machines makes use of mach-omap2 directory.
So just add one entry so that there is no need to patch this file
for any OMAP2+ devices.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>