mtd: devices: Provide header for shared OPCODEs and SFDP commands

JEDEC have helped to standardise a great deal of the commands which
can be issued to a Serial Flash devices. Many of the Serial Flash
Discoverable Parameters (SFDP) commands are generic across devices.
This patch provides a shared point where these commands can be
defined.

Suggested-by: Mark Brown <broonie@kernel.org>
Acked-by Angus Clark <angus.clark@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>