platform_data/mlxreg: Add support for complex attributes

Add new field 'regnum' to the structure 'mlxreg_core_data' to specify
the number of registers occupied by multi-register attribute.

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