qcom: clock: Allow enable always on clocks that are not rate settable

Some clocks do not support set rate (e.g. gate clocks, bus
interface clocks) but might still need to be kept always on.
Therefore, make the initial configuration rate optional when
enable the always on clocks.

Also, the requirement that a clock need to be set rate before
enabled can be captured with warning from the clk_prepare_enable
call. And corresponding clients can fix any misuse in their code.

Change-Id: I54cc1e708f940ae14ec3cadd853671a9b1344d98
Signed-off-by: Tianyi Gou <tgou@codeaurora.org>