i2c: mux: Fix the crash when the i2c-arbitrator node is present

i2c: mux: Fix the crash when the i2c-arbitrator node is present

Observing the crash when we add the i2c-arbitrator node in the device
tree as per the DT bindings. The issue is with the child node of
i2c-arbitrator@72 i.e., i2c@f1950000->i2c-arbitrator@72->i2c-arb, as the
arbitrator uses the uclass of mux(UCLASS_I2C_MUX) and the mux uclass driver
checks for the "reg" property using the i2c_mux_child_post_bind() function,
if it won't find the "reg" property it will return -EINVAL which is leading
to the crash.
So, add the logic to check whether the  child node has the "reg" property,
if the "reg" property exists then read the "reg" and update the channel.

https://www.kernel.org/doc/Documentation/devicetree/bindings/i2c/i2c-arb.txt

Signed-off-by: Venkatesh Yadav Abbarapu <venkatesh.abbarapu@amd.com>
Reviewed-by: Heiko Schocher <hs@denx.de>