soc: imxrt11xx: Clean up LDO configs

The LDO config should not be forcefully selected at SOC level. Instead,
use soft default y so that board definition can unset it if desired.

Also, in the soc.c, the LDO code should be on both the rt1160 and
rt1170, so the definitions should exist, and ifdef is not needed. So can
switch to IS_ENABLED to decrease configuration complexity of the source
code.

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