asmc: Add support for MacBook5,5

This is a slightly refined version of a patch[0] by FreeBSD Forums
user "tmcclean".

[0]https://forums.freebsd.org/threads/success-installing-freebsd-13-0-on-macbook-pro-5-5.80458/

Reviewed by: adrian, emaste, imp
Differential Revision: https://reviews.freebsd.org/D37900
MFC after: 1 month