mmc: sdhci-msm: Optionally wait for signal level changes
Not all instances of the SDCC core supports changing signal voltage and
as such will not generate a power interrupt when the software attempts
to change the voltage. This results in probing the eMMC on some devices
to take over 2 minutes.
Check that the SWITCHABLE_SIGNALING_VOLTAGE bit in MCI_GENERICS is set
before waiting for the power interrupt.