SPIM3 buffer must be in first 64kB of RAM

[dhalbert]

Fixes #3247 and fixes #3250 (@DavePutz, please test, thanks)
Fixes #3147
Fixes #2946

#3250: I believe this error is caused by #3147, which notes that the SPIM3 buffer is misplaced. To put it in its own 8kB region, the SoftDevice RAM area had to shrink from 64kB to 56kB. This was done by shrinking BLEIO_HVN_TX_QUEUE_SIZE from 9 to 5. The number of connections did not need to be reduced from its present 5.

For nRF52833, the special SPIM3 buffer is not necessary, because the SPIM3 peripheral does not suffer from the same hardware bug as on the nRF52840. So the SPIM3 buffer is disabled by default on nRF52833 builds.

I also cleaned up the mpconfigboard.mk files for nRF52833 and some nRF52840 boards. The nRF52833 boards no longer need to special-case removing the SPIM3 buffer. All nRF boards now have two I2C peripherals available, since SPIM3 can be used, so some needless definitions were removed.

The .ld template file was cleaned up for nRF: new RAM region start addreses and sizes are now defined and used. Sanity checking now takes places in mpconfigport.h to make sure the region sizes and locations make sense.

@xobs: I do not have nRF52833 boards to test, so could you test the reorganization of the settings? I moved some definitions around in mpconfigport.h which should have removed the need to do CFLAGS defines in the nRF52833 mpconfigboard.mk files. In particular the 32kB SD RAM size should still work. Thanks.

[dhalbert]

@DavePutz tested and this fixes #3247 and #3250. -- thanks!

[tannewt]

This looks good to me!