When it was added in r314636, AMD Thresholding was hardcoded to only
bank 4 (Northbridge) for some reason.  However, even on family 10h the
MCAx_MISC register Valid/Present bits determine whether thresholding is
supported on that bank.

Expand thresholding support to monitor all monitorable banks.  This
simplifies some of the logic and makes it more consistent with our Intel
CMCI support.

Reviewed by:	markj (earlier version)
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D12321