x86/microcode: Add explicit CPU vendor dependency

Add an explicit dependency to the respective CPU vendor so that the
respective microcode support for it gets built only when that support is
enabled.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/8ead0da9-9545-b10d-e3db-7df1a1f219e4@infradead.org