clk: sprd: add mux clock support

This patch adds clock multiplexor support for Spreadtrum platforms,
the mux clocks also can be found in sprd composite clocks, so
provides two helpers that can be reused later on.

Signed-off-by: Chunyan Zhang <chunyan.zhang@spreadtrum.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>