[qemu] Earlgrey 1.0.0 `i2c` implementation

[moidx]

Implement i2c functionality compatible with Earlgrey 1.0.0.

Deprioritize loopback support.

[moidx]

@luismarques: are I2C devices going to be emulated, or do we also need to support physical connections.

@amit-kumarh: Fairly sure emulation devices are ok. Will look into this and get back to you.

[amit-kumarh]

@luismarques Confirmed that yes, I2C emulated devices are OK for this, so long as we're able to communicate with the chip over I2C via opentitanlib in #27909 , or some other API to send I2C transactions to the chip from our test cases

cc @moidx

[ziuziakowska]

I2C implementation has been merged: lowRISC/qemu#162, with an initial test with emulated devices in Opentitan: #28134.

I2C transport on the QEMU side is in lowRISC/qemu#190, which will be used by opentitantool when complete.