microblaze: Support timer on AXI lite

New microblaze systems uses two buses. One for memories
and flashes and the second for low-speed peripherals
which can run on different CLK. This is the reason
why the kernel is trying to read clock-frequency
directly from node. If there is then the kernel will
work with it. If not then cpu CLK is used.

Signed-off-by: Michal Simek <monstr@monstr.eu>