serial: mxc: Add compatibles to NXP's serial driver (for imx21 and imx53)

serial: mxc: Add compatibles to NXP's serial driver (for imx21 and imx53)

This patch fixes not enabled uart2 (and hence serial console) on i.MX53
devices. After following commit 1d255904c306 ("ARM: dts: imx: imx53:
Synchronize iMX53 DT with Linux") from the uart2 compatible the
'fsl,imx7d-uart' has been removed (which was correct).

However, the root cause of the problem was the commit 98d62e618bb9 ("arm:
imx: add i.MX53 Beckhoff CX9020 Embedded PC") which introduced this
compatible.

Moreover, without this patch all i.MX53 UARTs are not usable as neither
'fsl,imx53-uart' nor 'fsl,imx21-uart' are in compatible IDs in
drivers/serial/serial_mxc.c file.

The fix is to add 'fsl,imx53-uart' and 'fsl,imx21-uart' as compatibles
for the aforementioned serial driver (those are also defined in the Linux
kernel).

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Acked-by: Marek Vasut <marex@denx.de>