microblaze: timer: Use CLKSRC_OF initialization

Simplify timer initialization and prepare the driver
for moving to drivers/clocksource folder.
Also remove system-timer property from binding because
the name is too generic.

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

diff --cc arch/microblaze/Kconfig
Simple merge

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

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