ARM i.MX53: Fix UART pad configuration

The current default pad configuration for UART RX and TX pads sets a 360k
pull-down and writes 1 to a reserved bit (1 << 0). It doesn't seem right to
me that in idle state, the UART has to keep the signal high against a
pull-down resistor.

This patch instead sets a 100k pull-up, which incidentally corresponds to the
register reset value for all but one (MX53_PAD_KEY_ROW0__UART4_RXD_MUX) pad,
and removes the write to the reserved bit.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>