clk: Extend struct clk to provide clock type agnostic flags

This commit extends the struct clk to provide information regarding the
flags related to this devices.

Those flags are clk device agnostic and indicate generic features
(like e.g. CLK_GET_RATE_NOCACHE - the need to always recalculate the rate).

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>