serial: lpuart: restructure lpuart driver

Drop CONFIG_LPUART_32B_REG.
Move the register structure to a common file include/fsl_lpuart.h
Define lpuart_serial_platdata structure which includes the reg base and flags.
For 32Bit register access, use lpuart_read32/lpuart_write32 which handles
big/little endian.
For 8Bit register access, still use the orignal code.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by : Stefano Babic <sbabic@denx.de>
Cc: Bhuvanchandra DV <bhuvanchandra.dv@toradex.com>
Cc: York Sun <york.sun@nxp.com>
Cc: Shaohui Xie <Shaohui.Xie@nxp.com>
Cc: Alison Wang <b18965@freescale.com>