boards: nxp: convert board sys_inits to hooks

Convert the nxp boards that have general init functions
as SYS_INITs to use board_early_init_hook instead.

Also standardize selection of CONFIG_BOARD_EARLY_INIT_HOOK in the
Kconfig file across NXP boards.

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