clk: gate: add CLK_GATE_HIWORD_MASK

In Rockchip Cortex-A9 based chips, they don't use paradigm of
reading-changing-writing the register contents.  Instead they
use a hiword mask to indicate the changed bits.

When b1 should be set as gate, it also needs to indicate the change
by setting hiword mask (b1 << 16).

The patch adds gate flag for this usage.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>