platform/mellanox: mlxreg-hotplug: Modify to use a regmap interface

Restructure mlxreg header for unification of hotplug item definitions.

Unify hotplug items to allow any kind of item (power controller, fan
eeprom, psu eeprom, asic health) in common way.

Use a hardware independent regmap interface, enabling the support of
hotplug events over programmable devices attached to different bus
types, such as I2C, LPC, or SPI. Add a device node to the
mlxreg_core_data structure.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
[dvhart: spelling corrections, refactor device node introduction]
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>