The 'flash-id' property defines the number of QSPI's dedicated GPIO
bank (and flash ID), which will be used to communicate with flash
memory.
For example, on stm32h7xx it is possible to use 'quadspi_bk1_*' and
'quadspi_bk2_*' set of pins, so one may need to select between them
when required.
By default - pins from 'quadspi_bk1_*' bank are used, so the
'flash-id = <2>;' property, when your use case (e.g. PCB design)
requires it, forces usage of 'quadspi_bk2_*' pins.
Signed-off-by: Lukasz Majewski <lukma@denx.de>