i2c: xiic: Implement power management

Enable power management. This patch enables the clocks before transfer
and disables after the transfer. It also adds the clock description.

Signed-off-by: Shubhrajyoti Datta <shubhraj@xilinx.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>