clk: sandbox: Adjust clk-mux.c to emulate reading divider value from HW

The generic mux clock code for CCF requires reading the clock multiplexer
value from HW registers. As sandbox by design has readl() as no-op it was
necessary to provide this value in the other way.

The new field in the mux structure (accessible only when sandbox is run)
has been introduced for this purpose.

Signed-off-by: Lukasz Majewski <lukma@denx.de>