zynq: remove use of CLKDEV_LOOKUP

The Zynq support in mainline does not (yet) make use of any of the
generic clk or clk lookup functionality.  Remove what is upstream for
now, until the out-of-tree implementation is in suitable form for
merging.

An important side effect of this patch is that it allows the building of
a Zynq kernel without running into unresolved symbol problems:

   drivers/built-in.o: In function `amba_get_enable_pclk':
   clkdev.c:(.text+0x444): undefined reference to `clk_enable'
   drivers/built-in.o: In function `amba_remove':
   clkdev.c:(.text+0x488): undefined reference to `clk_disable'
   drivers/built-in.o: In function `amba_probe':
   clkdev.c:(.text+0x540): undefined reference to `clk_disable'
   drivers/built-in.o: In function `amba_device_add':
   clkdev.c:(.text+0x77c): undefined reference to `clk_disable'
   drivers/built-in.o: In function `enable_clock':
   clkdev.c:(.text+0x29738): undefined reference to `clk_enable'
   drivers/built-in.o: In function `disable_clock':
   clkdev.c:(.text+0x29778): undefined reference to `clk_disable'
   drivers/built-in.o: In function `__pm_clk_remove':
   clkdev.c:(.text+0x297f8): undefined reference to `clk_disable'
   drivers/built-in.o: In function `pm_clk_suspend':
   clkdev.c:(.text+0x29bc8): undefined reference to `clk_disable'
   drivers/built-in.o: In function `pm_clk_resume':
   clkdev.c:(.text+0x29c28): undefined reference to `clk_enable'
   make[2]: *** [vmlinux] Error 1
   make[1]: *** [sub-make] Error 2
   make: *** [all] Error 2

In addition, eliminate Zynq's "use" of the versatile platform, as it is
no longer needed.  As Nick Bowler points out:

   For the record, I think this was introduced by commit 56a34b03
   ("ARM: versatile: Make plat-versatile clock optional") which forgot to
   select PLAT_VERSATILE_CLOCK on Zynq.  This is not all that surprising,
   because the fact that Zynq "uses" PLAT_VERSATILE is secretly hidden in
   the Makefile.

   Nevertheless, the only feature from versatile that Zynq needed was the
   clock support, so this patch should *also* delete the secret use of
   plat-versatile by removing this line from arch/arm/Makefile:

      plat-$(CONFIG_ARCH_ZYNQ)      += versatile

Signed-off-by: Josh Cartwright <josh.cartwright@ni.com>
Cc: John Linn <john.linn@xilinx.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Michal Simek <michal.simek@xilinx.com>