mmc: omap: Add erase capability

This patch adds the erase capability to OMAP1/OMAP2420 MMC driver. Idea is
the same than in commit 93caf8e6 ("omap_hsmmc: add erase capability") that we
disable the data timeout interrupt for erases.

Signed-off-by: Jarkko Nikula <jarkko.nikula@bitmer.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Chris Ball <chris@printf.net>