mmc: core: Set non-default Drive Strength via platform hook

Non default Drive Strength cannot be set automatically.  It is a function
of the board design and only if there is a specific platform handler can
it be set.  The platform handler needs to take into account the board
design.  Pass to the platform code the necessary information.

For example:  The card and host controller may indicate they support HIGH
and LOW drive strength.  There is no way to know what should be chosen
without specific board knowledge.  Setting HIGH may lead to reflections
and setting LOW may not suffice.  There is no mechanism (like ethernet
duplex or speed pulses) to determine what should be done automatically.

If no platform handler is defined -- use the default value.

Signed-off-by: Philip Rakity <prakity@marvell.com>
Reviewed-by: Arindam Nath <arindam.nath@amd.com>
Signed-off-by: Chris Ball <cjb@laptop.org>