mlxsw: Query UTC sec and nsec PCI offsets and values

Query UTC sec and nsec PCI offsets during the pci_init(), to be able to
read UTC time later.

Implement functions to read UTC seconds and nanoseconds from the offset
which was read as part of initialization.

Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>