[SRU][X,B,D] backport DIMLIB (lib/dim/) to pre-5.2 kernels

[Kamal Mostafa]

BugLink: https://bugs.launchpad.net/bugs/1852637

The mellanox and broadcom drivers contained a "dynamic interrupt
moderation" feature (as early as v5.0) which was broken out into a
common kernel library 'DIMLIB' in lib/dim/ circa mainline v5.2.

Backported drivers may now need DIMLIB to be available in pre-5.2
kernels (specifically, the latest upstream version of
net/ethernet/amazon/ena does).

Pull req's to follow ...

For Disco:
    Apply all the mainline commits from v5.2 which extract DIMLIB from
    mellanox/broadcom and wire those drivers to use DIMLIB (plus a few
    later DIMLIB fixes). I.e. make Disco work the same as > 5.2 kernels;
    no significant functional change to those drivers.

For Bionic and Xenial:
    Import the lib/dim/ directory from mainline (plus a couple porting
    prerequisites), but do *not* make mellanox/broadcom use it. I.e. no
    functional change at all.

 -Kamal