machine: use larger SPI MAXCNT on nrf52833 and nrf52840

These chips have a larger upper limit for the DMA transfer than the
nrf52832. For best performance, we should be splitting the transfer in
as large blocks as possible on the given hardware.

src/machine/machine_nrf52.go

@@ -69,3 +69,5 @@ const eraseBlockSizeValue = 4096
 func eraseBlockSize() int64 {
 	return eraseBlockSizeValue
 }
+
+const spiMaxBufferSize = 255 // from the datasheet: TXD.MAXCNT and RXD.MAXCNT