platform/x86: mlx-platform: Fix power cable setting for msn21xx family

Add dedicated structure with power cable setting for Mellanox msn21xx
family. These systems do not have a physical device for the power unit
controller. When the power cable is inserted or removed, the relevant
interrupt signal is handled, the status is updated, but no device is
associated with the signal.

Add definition for interrupt low aggregation signal. On system from
msn21xx family, low aggregation mask should be removed in order to allow
signal to hit CPU.

Fixes: 6613d18e

 ("platform/x86: mlx-platform: Move module from arch/x86")
Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>