i2c: mux: Allow muxes to work as children of i2c bus without i2c-parent

i2c: mux: Allow muxes to work as children of i2c bus without i2c-parent

For mux check if the parent is already a device of UCLASS_I2C and if yes
just use that. Otherwise see if someone specified an i2c-parent phandle.
This mimics the behavior found in the Kernel, as it removes the
requirement to explicitly specify a i2c-parent phandle.

Signed-off-by: Moritz Fischer <moritz.fischer@ettus.com>
Cc: Heiko Schocher <hs@denx.de>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: u-boot@lists.denx.de