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>

drivers/i2c/busses/Kconfig

@@ -378,7 +378,7 @@ config I2C_BLACKFIN_TWI_CLK_KHZ
 
 config I2C_CADENCE
 	tristate "Cadence I2C Controller"
-	depends on COMMON_CLK
+	depends on ARCH_ZYNQ
 	help
 	  Say yes here to select Cadence I2C Host Controller. This controller is
 	  e.g. used by Xilinx Zynq.