drivers: lpadc: Move SOC code out of driver

To be consistent with the current NXP clocking scheme,
move the LPADC clocking code to the SOC files where
all of the other peripheral clocking is done.

Also remove any other SOC-specific code to the
respective SOC file and out of this driver.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>