ARM: imx: clk-imx27: Do not register peripheral clock for SSI

imx ssi block has two types of clocks:

- ipg: bus clock, the clock needed for accessing registers.
- per: peripheral clock, the clock needed for generating the bit rate.

Currently ssi driver only supports slave mode and thus need only to handle
the ipg clock, because the peripheral clock comes from the master codec.

Only register the ipg clock and do not register the peripheral clock for ssi

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>