boards: mimxrt595evk: Setup the regulator for Run mode

Setup the appropriate voltage in the PMIC based on the
core frequency.
Add labels to the regulators for easy access to the nodes.
Add CONFIG_REGULATOR to the defconfig so we can setup the
PMIC voltages during platform startup.

Signed-off-by: Mahesh Mahadevan <mahesh.mahadevan@nxp.com>