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>