MAINTAINERS: OMAP2+ support, add tps65218-pwrbutton

The entry for the pwrbutton driver seems to be missing. Add it to the
list for OMAP2+ SUPPORT.

Signed-off-by: Markus Schneider-Pargmann <msp@baylibre.com>
Signed-off-by: Jerome Neanne <jneanne@baylibre.com>