serial: lpuart: add i.MX7ULP support

Add i.MX7ULP support.
The buadrate calculation on i.MX7ULP is different,so add a new setbrg
function for i.MX7ULP.
Add a enum lpuart_devtype for runtime check for different platforms.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: 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>