Documentation/ABI: Add new attribute for mlxreg-io sysfs interfaces

Add documentation for the new attributes for systems of type MQM87xx,
MSN34xx, MSN37xx. It includes the attributes related to ComEx resets,
system platform reset, JTAG interface enabling, additional CPLD device.
All the above system are equipped with ComEX board, have JTAG selector
and extra CPLD.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
[dvhart: Change kernel version from 5.0 to 4.21 until we know for sure]
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>