Add a common clock driver for basic gpio controlled clock multiplexers.
This driver can be used for devices like 5V41068A or 831721I from IDT
or for discrete multiplexer circuits. The 'select' pin selects one of
two parent clocks.

Cc: Jyri Sarha <jsarha@ti.com>
Signed-off-by: NSergej Sawazki <ce3a@gmx.de>
[sboyd@codeaurora.org: Fix error paths to free memory and do it
in the correct order]
Signed-off-by: NStephen Boyd <sboyd@codeaurora.org>