platform/x86: mlx-platform: Add support for new capability register

Add support for capability register, which contains information about
the number of PS units equipped on the system and about minimum I2C
frequency supported by the all system's I2C devices.
Utilization of this register allows to avoid necessity of providing new
system description, in case it differs in number of PS units or in
minimal I2C frequency.

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