sysutils/devcpu-data-amd: Download microcode from ucode repo

• Download AMD microcode firmware from git.kernel.org.

• Make updates slightly simpler by referencing ${DISTFILES} rather than repeating the contents.

• Expand pkg-descr.

• Group and sort standard bsd.port.mk variables.

Reported by: sbruno (maintainer)
Approved by: sbruno (maintainer)
Differential Revision: https://reviews.freebsd.org/D32935