[ot] docs/opentitan: earlgrey: document SPI Passthrough flash options

Signed-off-by: Alice Ziuziakowska <[email protected]>

Author: ziuziakowska

docs/opentitan/earlgrey.md

@@ -239,7 +239,7 @@ See documentation in [`keymgr.md`](./keymgr.md).
   MTD bus 0 is assigned to the SPI0 Host controller and MTD bus 1 is assigned to the SPI1 Host
   controller. See also Embedded Flash controller section.
 
-* `-global ot-earlgrey-board.spiflash<bus>=<flash_type>` should be used to instanciate a SPI
+* `-global ot-earlgrey-board.spiflash<bus>=<flash_type>` should be used to instantiate a SPI
   dataflash device of the specified type to the first device (/CS0) of the specified bus.
   Any SPI dataflash device supported by QEMU can be used. To list the supported devices, use
   `grep -F 'INFO("' hw/block/m25p80.c | cut -d'"' -f2`
@@ -258,6 +258,18 @@ See documentation in [`keymgr.md`](./keymgr.md).
   best SPI Host transfer performances, but decreases accuracy of the SPI Host clock settings. Time
   should be specified in ns, and defaults to 0 that indicates automatic SPI bus clock management.
 
+### SPI Device
+
+* `-global ot-earlgrey-board.spiflash_passthrough=<flash_type>` should be used to instantiate a
+  downstream SPI dataflash device of the specified type for use by OT SPI Device in Passthrough
+  mode. The list of supported devices is the same as for SPI Host.
+
+* `-drive if=mtd,bus=3,file=<filename>,format=raw` should be used to specify a PATH to a QEMU RAW
+  image file used as the backing storage for the downstream flash.
+
+  MTD bus 3 is assigned to the downstream flash device accessible through OT SPI Device in
+  Passthrough mode. See the SPI Host and Embedded Flash sections for other bus assignments.
+
 ### UART
 
 See documentation in [`uart.md`](./uart.md).