sandbox: clk: add clk enable/disable test code

Since we added clk enable_count and prograte clk child enabling
operation to clk parent, so add a new function sandbox_clk_enable_count
to get enable_count for test usage.

And add test code to get the enable_count after we enable/disable
the device clk.

Signed-off-by: Peng Fan <peng.fan@nxp.com>