ARM: zynq: Add missing i2c get_rate for fixing i2c SPL

The commit 'f48ef0d8' did break I2C
support because requesting the clock for the I2C ip-block isn't
supported during SPL.

To fixup this we add support requesting clocks for:
- i2c0
- i2c1

Signed-off-by: Hannes Schmelzer <hannes.schmelzer@br-automation.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>