mtd: spi-nor: add support for Macronix Octal flash

Follow patch <f6adec1a> (Allow using Micron mt35xu512aba
in Octal DTR mode).
Enable Octal DTR mode with 20 dummy cycles to allow running at the
maximum supported frequency for adding Macronix flash in Octal DTR mode.
 -https://www.mxic.com.tw/Lists/Datasheet/Attachments/7841/MX25LM51245G,%203V,%20512Mb,%20v1.1.pdf



Signed-off-by: JaimeLiao <jaimeliao.tw@gmail.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>