kernel: defconfig: increase squashfs threads for armv8

Change from 1 squashfs decompression thread per CPU core to 2 threads
per core for armv8 BMCs. This results in a significant speedup during
boot.

Limit this change to only armv8 BMCs since it causes a slowdown on some
armv7 BMCs.

Tested:
Time to multi-user.target on armv8 BMC went from  167.5s -> 148.75s
No change to armv7 kernel .config

Fusion-Link: http://fusion2/12975cad-b779-382f-a203-6742791ce634
Fusion-Link: http://fusion2/b6824802-7c98-3353-b4f4-9c91963d7dda
Fusion-Link: http://fusion2/3e26c565-fac7-3b11-a01a-3cb48ecc9b4f
Fusion-Link: http://fusion2/96492c7b-5689-3b4d-aaeb-d79ebf7a1e01
Google-Bug-Id: 340591490
Platforms-Affected: platform21, platform17, platform8, platform18, platform6, platform26
Change-Id: I86aff98b0d8ad4c65f3742c4d8d3bab26b3e5a63
Signed-off-by: Benjamin Fair <benjaminfair@google.com>