drivers/spi: add Macronix MX25U6435F and MX25L6495F support

Contributed by MXIC: http://www.coreboot.org/pipermail/coreboot/2014-October/078835.html

Change-Id: I07f872a5cbb2b0ea63794edb8fbca40d7856ce10
Author: Alex Lu <alexlu6@mxic.com.tw>
Signed-off-by: Idwer Vollering <vidwer@gmail.com>
Reviewed-on: http://review.coreboot.org/7194
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Tested-by: build bot (Jenkins)