platform/x86: mlx-platform: Add support for tachometer speed register

Add support for tachometer speed register for the next generation
systems MQMB7xx, MSN37xx, MSN34xx, MSN38xx.

All these systems support tachometer speed capability register.
This register is to be provided mlxreg-fan driver.

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