nrf: SPI flash and QSPI flash support

[dhalbert]

As used on existing SAMD Express boards.

I don't see an explicit issue for this, but maybe I missed it.

[hathach]

note to myself: The flash chip on

• feather52840 is GD25Q16CTIGR
• pca10056 it is MX25R6435F

[arturo182]

Remember to use nRFx :D

[hathach]

Will try to remember it :D. I went through the time when Nordic sdk upgrade requires massive amount of work for migration. So I often to write the code in its generic form. The new nrfx seems to do a great job :D

[hathach]

QSPI on pca10056

[hathach]

doing a bit of testbench, is the speed Ok
clock 32 Mhz

clock 8 Mhz

[ladyada]

cool!

[hathach]

The read speed is similar to cpx test, but the write speed is not that great. I will do a bit more tweaks to see if we could improve it.

[dhalbert]

The write code -might- need work. It seems slow on SAMD51 (writing a large file takes a long time), and maybe there's a little bug somewhere.

[tannewt]

Write speed is erase limited since we don't do any fancy block allocations. I would love to do balancing that erases after a write but that only helps for a single block.

[tannewt]

I added this with 97bc951