platform/mellanox: mlxreg-io: Add support for complex attributes

Add support for attributes composed from few registers.
Such attributes could occupy from 2 to 4 sequential registers.
For word size register space complex attributes can occupy up to two
registers, for byte size - up to four. These attributes can carry, for
example, CPLD or FPGA versioning, power consuming info, etcetera.
Such registers contain read-only data.

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