mtd: cfi: Support early CFI fixups

Some CFI devices need fixups that affect the number of chips detected,
but the current fixup infrastructure (struct cfi_fixup and cfi_fixup())
does not cover this situation.

Introduce struct cfi_early_fixup and cfi_early_fixup() to fill the void.

Signed-off-by: Aaron Sierra <asierra@xes-inc.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>