MLK-21057 serial: imx: Restore proper init of shadow registers

Upstream commit 3a0ab62f ("serial: imx: implement shadow registers
for UCRx and UFCR") made the imx_uart_readl/writel functions use a
"cache" to avoid reads/writers to hardware for some registers.

Those values need to be initialized once at probe time using real reads
from hardware; using imx_uart_readl for this is obviously non-functional

Fixes: 372312842fe7 ("tty: serial: imx: use imx_uart_writel() and imx_uart_readl()")

This is required for imx8mq to boot and likely fixes other things.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Acked-by: Andy Duan <fugang.duan@nxp.com>