msm: clock-generic: Add generic library for mux clocks

The clk_ops defined in this library can't be used directly, but they
implement majority of the code necessary to handle common clock types. The
HW specific operations are handled by calling ops implemented by the HW
specific driver. The clk structs defined in this library contain fields
that are not used by the helper library, but commonly needed by the HW
specific ops. They also provide a *priv field to store other uncommon data.

To start with, this file implement generic ops for a mux.

Change-Id: Ibc44b95934f402dbcbb1996d77a61d40d9e53f20
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>