Reduce ppc64's flashrom support to MTD only

[tlaurion]

As referred in Dasharo/flashrom#11 (comment) AST1100 can be removed, maybe others as well to leave only mtd support

[tlaurion]

#1251 is implementing this. Waiting for dasharo/flashrom to fix building issues for ppc64 in their branch so that module/flashrom Makefile works (works for x86, not for ppc64)

Closing since currently duplicate of #1251 (fixed on Heads side)