mmc: sdhci-of-aspeed: Handle optional controller reset

mmc: sdhci-of-aspeed: Handle optional controller reset

Get the optional reset line for the ASPEED SD controller during probe by
using devm_reset_control_get_optional_exclusive_deasserted().

This allows platforms such as AST2700, which require the SD controller
to be taken out of reset before use, to work with the existing driver.

Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ryan Chen <ryan_chen@aspeedtech.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>