EDAC, ghes: Expose groups as an additional layer

When an error is reported we map it to a dimm index. Now that we have
grouped the dimms by their type-16 'physical memory array', we can export
that as the top layer instead.

Add get_smbios_dimm_position() which searches for the provided dimm
handle and returns its group and dimm indexes. We deliberatly use this
to find the value we just allocated in ghes_edac_dmidecode() so that we
have confidence the same dimm will be found when an error is reported.

Signed-off-by: James Morse <james.morse@arm.com>