nxp_imx: Move i.MX RT PLL selects to Kconfig.soc

PLL configuration options for i.MX RT SoCs were added in commit
3fd25c64, but the selects were
incorrectly added to the SoC defconfig rather than Kconfig.soc. This
resulted in the PLL options not being configured and the part not
booting properly.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>