platform_data/mlxreg: Add presence register field for FAN devices

Add new field 'reg_prsnt' to the structure 'mlxreg_core_data' to
provide the number FAN drawers equpped within the system. The purpose
is to allow mapping between FAN drawers and FAN rotors (tachometer),
since FAN drawer can be eqipped with a few rotors.

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