boards: arduino: portenta_h7: Add support for external QSPI Flash.

Enabled the QuadSPI node and necessary pin muxing to enable interfacing
with the external flash chip on the Portenta H7 board. The jedec data was
validated with the samples/drivers/jesd216 project to extract the
JEDEC data.
Additionally added `flash` to the list of ignored tags as the flash driver
should not be built on the M4 CPU domain.

Signed-off-by: Rahul Arasikere <arasikere.rahul@gmail.com>