ARM: imx: move imx53-qsb audio codec clk lookup into DT

With device tree clk lookup support in place, we can move audio codec
clk lookup for ssi_ext1 into device tree now, so that imx53_qsb_init()
can be saved.

Since ssi_ext2 lookup is used nowhere, it gets removed together with
ssi_ext1 lookup from clk driver.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>