dm: sandbox: i2c: Add a new 'emulation parent' uclass

Sandbox i2c works using emulation drivers which are currently children of
the i2c device:

	rtc_0: rtc@43 {
		reg = <0x43>;
		compatible = "sandbox-rtc";
		emul {
			compatible = "sandbox,i2c-rtc";
		};
	};

In this case the emulation device is attached to i2c bus on address 0x43
and provides the Real-Time-Clock (RTC) functionality.

However this is not ideal, since every device on an I2C bus has a child
device. This is only really the case for sandbox, but we want to avoid
special-case code for sandbox.

A better approach seems to be to add a separate node on the bus, an
'emulation parent'. This can be given a bogus address (such as 0xff) and
hides all the emulators away. Then we can use a phandle to point from the
device to the correct emualtor, and only on sandbox. The code to find an
emulator does not interfere with normal i2c operation.

Add a new UCLASS_I2C_EMUL_PARENT uclass which allows finding an emulator
given a bus, and finding a bus given an emulator. This will be used in a
follow-on patch.

Signed-off-by: Simon Glass <sjg@chromium.org>

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