platform/mellanox: mlxreg-hotplug: Add support for new capability register

Add support for capability register, which is used for detection of the
actual number of interrupt capable components within the particular
group, supported by the specific system.
Such components could be for example the number of power units and
interrupts related to these units.
The motivation is to avoid adding a new code in the future in order to
distinct between the systems type supported different number of the
components like power supplies, FANs, ASICs, line cards.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>