clk: loongson2: Allow specifying clock flags for gate clock

Some gate clocks need to be supplied with flags, e.g., it may be
required to specify CLK_IS_CRTICAL for CPU clocks.

Add a field to loongson2_clk_board_info for representing clock flags,
and specify it when registering gate clocks. A new helper macro,
CLK_GATE_FLAGS, is added to simplify definitions.

Signed-off-by: Yao Zi <ziyao@disroot.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>