mtd: m25p80: Micron SPI uses Macronix-style 4-byte addressing

For SPI NOR flash that are larger than 128Mbit (16MiB), we need 4 bytes
of address space to reach the entire flash; however, the original SPI
flash protocol used only 3 bytes for the address. So far, the practice
for handling this has been either to use new command opcodes that are
defined to use 4 bytes for their address, or to use special
mode-switching command to configure all traditionally-3-byte-address
commands to take 4 bytes instead.

Macronix and Spansion developed two incompatible methods for
entering/exiting "4-byte address mode." Micron flash uses the Macronix
method (OPCODE_{EN4B,EX4B}), not the Spansion method.

This patch solves addressing issues on Micron n25q256a and provides the
ability to support other future Micron SPI flash >16MiB.

Quoting a Micron representative:

  "Majority of our NOR that needs 4-byte addressing (256Mb or 32MB and
   higher) enter and exit 4byte through B7h and E9h commands. The
   N25Q256A7xxx and N25Q512A7xxx parts do not support 4-byte addressing
   mode via B7h or E9h command."

They further clarified that those that don't support the enter/exit
opcodes (B7h/E9h) are manufactured specifically to come up by default in
4-byte mode. We don't need to treat those parts any diffently, as they
will discard the EN4B opcode as a no-op.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

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