microblaze: timer: Use generic sched_clock implementation

Remove sched_clock from the driver and use sched_clock_register
function.
Inspired-by:
"arch_timer: Move to generic sched_clock framework"
(sha1: 65cd4f6c99c1170bd0114dbd71b978012ea44d28)

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

diff --cc arch/microblaze/Kconfig
Simple merge

diff --cc arch/microblaze/kernel/timer.c
Simple merge