i2c: cadence: fix Kconfig dependency

During development, the driver first really needed to depend on
COMMON_CLK only. Later, it was switched to writel_relaxed, but it was
forgotten to update the dependencies, so build errors occured:

config: make ARCH=i386 allyesconfig

All error/warnings:

   drivers/i2c/busses/i2c-cadence.c: In function 'cdns_i2c_clear_bus_hold':
>> drivers/i2c/busses/i2c-cadence.c:168:3: error: implicit declaration
>> of function 'writel_relaxed' [-Werror=implicit-function-declaration]

Use a very safe dependency for now.

Signed-off-by: Wolfram Sang <wsa@the-dreams.de>