Resolved possible race conditions when accessing registers in different banks.
Added Tx ready semaphore when requesting packet to transmit (ENC is slow => access to it needs to be controlled).
Added setting of CS hold time based on Data sheet.
Added option to set the ENC28J60 to Full Duplex mode.
Addressed several ENC28J60 Erratas.
Restructured ENC28J60 example folder structure so the driver could be easily linked from different projects.
Extended the README to guide users of how to properly use the ENC28J60 chip.
Extended iperf example to include ENC28J60.
Closes https://github.com/espressif/esp-idf/issues/4747
Closes https://github.com/espressif/esp-idf/issues/7117
Closes https://github.com/espressif/esp-idf/issues/7156
