RMII TX on a 8b port

[shuchitak]

This has been implemented as part of #85. Further work is needed for validating the implementation.

• Calculate RMII_ETHERNET_IFG_DELAY_ADJUSTMENT_8b adjustments. test_time_tx checks this
• Possibly add timestamping in rmii_master_tx_pins_8b_asm and return values to XC. This should be OK as we can weave it into the end of the preamble and start of CRC where we have a few spare slots. May not be needed if we can close the above point
• Possible safeguarding against Error:relocation R_XCORE1_CP_REL6 is out of range in ASM
• Add support to 8b tx in rmii_phy.py
• Extend tests to cover 8b tx case
• Update docs to show how to use of 8b Tx ports, also additional memory usage of MAC (more than 1kB more due to LUT)
• Code review

[humphrey-xmos]

Plus, given teh 8b port for Eth-TX is shared with I2C. The boot time I2C config hold-off time, then handover port pins to the Ethernet MAC.