This change provides support for 4 IO read (via 4READ command) and
program (via 4PP). Flash memory pins SIO[0123], CLK and CS are used.
All of them are controlled by stm32 QSPI IP block.
The instruction code for fast reading as well as number of latency
cycles required are read from SFDP structure provided by flash memory.
The number of required read latency cycles when performing reading
is the sum of SPI-NOR memory mode bits and wait states (also named
as 'dummy cycles').
It also has been assumed that memory, which supports fast read (4READ),
also will support fast programming (4PP command) as this information is
not available in SFDP.
One also need to enable the QUAD IO support in board's device tree by
defining 'spi-bus-width = <4>' property. It is required as it may
happen that not all QSPI dedicated pins are used (for example only two
of them are available).
Signed-off-by: Lukasz Majewski <lukma@denx.de>
5599f1b3ab