ARM: 6246/1: mmci: support larger MMCIDATALENGTH register

The Ux500 variant has a 24-bit MMCIDATALENGTH register, as opposed to
the 16-bit one on the ARM version.

Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>