mtd: spi-nor: Emphasise which is the generic set_4byte_addr_mode() method

Rename (*set_4byte)() to (*set_4byte_addr_mode)() for a better
differentiation between the 4 byte address mode and opcodes.

Rename macronix_set_4byte() to spi_nor_set_4byte_addr_mode(), it will be
the only 4 byte address mode method exposed to the manufacturer drivers.

Here's how the manufacturers enter and exit the 4 byte address mode:
- eon, gidadevice, issi, macronix, xmc use EN4B/EX4B
- micron-st needs WEN. st_micron_set_4byte_addr_mode() will become
  a private method, as they are the only ones that need WEN before the
  EN4B/EX4B commands.
- newer spansion have a 4BAM opcode (this translates to a new, public
  command). Older spansion  flashes use the BRWR command (legacy in
  core.c -> spansion_set_4byte_addr_mode())
- winbond's method is hackish and may be reason for just a flash
  fixup hook -> private method

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Vignesh Raghavendra <vigneshr@ti.com>