jedec_dimm(4): Add manufacturing year and week.

DDR3 and DDR4 encode the week and year that the DIMM was manufactured,
as a pair of two-digit binary-coded decimal values. Read the values, and
report them as (uint8_t)s.

Reviewed by: imp, jhb
MFC after: 1 week
Sponsored by: Panasas
Differential Revision: https://reviews.freebsd.org/D39795