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>

drivers/clk/zynq/Makefile

 # Zynq clock specific Makefile
 
-obj-$(CONFIG_ARCH_ZYNQ)	+= clkc.o pll.o
+obj-y	+= clkc.o pll.o