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

Also according to RM, the Receive RX FIFO Enable (RXFE) field in LPUART
FIFO register is bit 3, so this definition should change to 0x08 as done
for i.MX8. It needs also to set baudrate the same way as i.MX8 does.
Signed-off-by: NGiulio Benetti <giulio.benetti@benettiengineering.com>