mmc: core: Share internal function to set initial signal voltage

Move the corresponding code for setting the initial signal voltage, from
mmc_power_up() into a new function, mmc_set_initial_signal_voltage().

Make the function internally available to the mmc core, as to allow the
following changes to make use of it.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Quentin Schulz <quentin.schulz@bootlin.com>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>