drivers: flash: spi_nor: Add fast read support

Most SPI NOR flash devices support a "fast read" command which uses
dummy bits between the address and the start of the data transfer. In
many cases, the maximum SPI clock speed of the device is lower for the
regular read command due to the limited time between the address and
data phases, so using the fast read command will remove this restriction
and allow for faster transfers.

Add a device tree flag to indicate that fast reads should be used for
the device.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>