MMC: Max blocks value adjustable

The maximum blocks value was hardcoded to 65535 due to a 16 bit
register length.  The value can change for different platforms.
This patch makes the default the current value of 65535, but it
is configurable for other platforms.

Signed-off-by: Matt Waddel <matt.waddel@linaro.org>
Signed-off-by: Andy Fleming <afleming@freescale.com>