ARM: clk-imx25: Fix SSI clock registration

SSI block has two types of clock:

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 only need 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>
Cc: stable@vger.kernel.org