This driver fails to build when HAVE_CLK and COMMON_CLK are disabled:
x86_64-linux-ld: vmlinux.o: in function `mgb4_remove':
mgb4_core.c:(.text+0x1915e8c): undefined reference to `clkdev_drop'
x86_64-linux-ld: mgb4_core.c:(.text+0x1915e98): undefined reference to `clk_hw_unregister'
Add a Kconfig dependency to enforce a clean build.