configurations: Add ASRock SPC621D8HM3 board

This is a half-width, AST2500-based, single-socket Xeon server board
employed in the ASRock n3.xlarge.x86 server.

Tested: All motherboard-based sensors

configurations: Add ASRock SPC621D8HM3 board

This is a half-width, AST2500-based, single-socket Xeon server board
employed in the ASRock n3.xlarge.x86 server.

Tested: All motherboard-based sensors work as expected in an ASRock
n3.xlarge.x86 server.

Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Change-Id: If1bacace1637761ddabbc0e2daf6803d4475e0f0

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