arm: zynq: Enable SPL_CLK only if SPL is enabled

Setup proper dependency in Kconfig for SPL_CLK.
If SPL is not enabled, SPL_CLK shouldn't be selected.

Cc: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>