iMX: Fix build for iMX53

Commit fad10708 fixed the wrong test for MX51
as the MX51 addresses are wrong for MX50 and MX53 but now it's MX51 only,
UART_PADDR is not defined anymore when building for MX50/MX53.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Tested-by: Steev Klimaszewski <steev@genesi-usa.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>