serial_lpuart: Update lpuart driver to support i.MX8

serial_lpuart: Update lpuart driver to support i.MX8

Add i.MX8 compatible string and cpu type support to lpuart driver,
to use little endian 32 bits configurations.

Also, according to RM, the Receive FIFO Enable (RXFE) field in LPUART
FIFO register is bit 3, so this definition should change to 0x08
(not 0x40) for i.MX8, otherwise the Receive FIFO is not disabled.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Anatolij Gustschin <agust@denx.de>
Cc: Stefano Babic <sbabic@denx.de>