Xilinx: ARM: timer: Remove useless conditionals

When porting the timer to use the common clock framework
it was assumed it is possible to compile a Zynq kernel
w/ or w/o COMMON_CLK defined and conditional compilation had
been added accordingly.
Turns out this assumption was wrong and building for Zynq always
defines COMMON_CLK rendering the associated conditionals useless.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>