ARM: imx51.dtsi: fix PATA device clock

Commit 718a3500 (ARM: i.MX51: Add PATA support) adds pata support to the
imx51.dtsi file and is using clock 161. The problem is that the right
clock is 172, according to commit 5d530bb0 (ARM: i.MX5: Add PATA and SRTC
clocks).  Using the clock 172 makes things work again (and kills a nasty
system freeze).

Tested-by: Steev Klimaszewski <steev@gentoo.org>
Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>