mtd: spi-nor: unify read opcode variants with ST SPI FSM

serial_flash_cmds.h defines our opcodes a little differently. Let's
borrow its naming, since it's borrowed from the SFDP standard, and it's
more extensible.

This prepares us for merging serial_flash_cmds.h and spi-nor.h opcode
listing.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Acked-by: Huang Shijie <b32955@freescale.com>