ARM: display5: Remove U_BOOT_DEVICE definition of serial_mxc

Before the wide DM/DTS adoption in the U-Boot proper, the display5
has been using only DM_SERIAL to provide serial console in
pre-relocation.

After moving to full DM/DTS adoption in the U-Boot proper the
U_BOOT_DEVICE definition is not needed anymore, as it has been
replaced with udevice creation from provided DTS description.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>