i2c: cadence: Implement save restore

The zynqmp platform now supports chip-off so the registers can
lose context.
Implement save restore for i2c module.
Since we have only a couple of registers
an unconditional restore is done.

Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@xilinx.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>