ARM: zynq: move arm-specific sys_timer out of ttc

Move the sys_timer definition out of ttc driver and make it part of the
common zynq code.  This is preparation for renaming and COMMON_CLK
support.

Signed-off-by: Josh Cartwright <josh.cartwright@ni.com>
Tested-by: Michal Simek <michal.simek@xilinx.com>