mmc: core: Extend mmc_of_parse() to check for mmc-ddr-3_3v

When mmc_of_parse() finds the binding, it sets the mmc cap,
MMC_CAP_3_3V_DDR, which informs the core whether eMMC DDR at 3.3V I/O is
supported by the mmc host.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Tested-by: Jan Glauber <jglauber@cavium.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>