platform_data/mlxreg: Add capability field to core platform data

Add capability field to "mlxreg_core_platform_data" structure.
The purpose of this register is to provide additional info to platform
driver through the atribute related capability register.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>