i2c: pca-platform: use device_property_read_u32

Use device_property_read_u32 instead of of_property_read_u32_index to
lookup the "clock-frequency" property.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>