clk: zynq: remove redundant $(CONFIG_ARCH_ZYNQ) in Makefile

Kbuild descends into drivers/clk/zynq/ only when CONFIG_ARCH_ZYNQ
is enabled.  (see drivers/clk/Makefile)

$(CONFIG_ARCH_ZYNQ) in drivers/clk/zynq/Makefile always evaluates
to 'y'.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>