Documentation/ABI: Add new attribute for mlxreg-io sysfs interfaces

The systems of type MSN21xx, MSN201x have a slight difference from the
default reset cause bits description, it uses reset_hotswap_or_halt
instead of reset_hotswap_or_wd. New attribute is documented.

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