mmc: add a function to get regulators, supplying card's power

Add a function to get regulators, supplying card's Vdd and Vccq on a
specific host. If a Vdd supplying regulator is found, the function checks,
whether a valid OCR mask can be obtained from it. The Vccq regulator is
optional. A failure to get it is not fatal.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Reviwed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>