clock-local2: Enhance gate_clk to handle delay and rates

Some gate clocks require delay after writing to enable bit. Add
delay_us to account for the delay. Gate clocks should also handle
clk_round_rate and clk_get_rate by passing the request to its parent.

Change-Id: I84622f3675566e5c19d1422123d643ce391a1b63
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>