ptp: ocp: add COMMON_CLK dependency

Without CONFIG_COMMON_CLK, this fails to link:

arm-linux-gnueabi-ld: drivers/ptp/ptp_ocp.o: in function `ptp_ocp_register_i2c':
ptp_ocp.c:(.text+0xcc0): undefined reference to `__clk_hw_register_fixed_rate'
arm-linux-gnueabi-ld: ptp_ocp.c:(.text+0xcf4): undefined reference to `devm_clk_hw_register_clkdev'
arm-linux-gnueabi-ld: drivers/ptp/ptp_ocp.o: in function `ptp_ocp_detach':
ptp_ocp.c:(.text+0x1c24): undefined reference to `clk_hw_unregister_fixed_rate'

Fixes: a7e1abad

 ("ptp: Add clock driver for the OpenCompute TimeCard.")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>