drivers: sdhc: imx_usdhc: move voltage selection to separate function

Some USDHC IP instances do not have the voltage control bit present, as
they can only operate at 3.3V. Move code to select 1.8V mode into a
separate helper, and guard the call to UDSHC_SelectVoltage() behind a
feature macro from MCUX SDK.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>