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.

Cc: Sahitya Tummala <stummala@codeaurora.org>
Cc: Vijay Viswanath <vviswana@codeaurora.org>
Fixes: c0309b38 ("mmc: sdhci-msm: Add sdhci msm register write APIs which wait for pwr irq")
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Tested-by: Luca Weiss <luca@z3ntu.xyz>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>