spi: sh-msiof: Document hardware limitations related to chip selects

Guide users to maintain the proper balance between native and GPIO chip
selects.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/sh-msiof.txt

@@ -41,6 +41,16 @@ Optional properties:
 			   0: MSIOF_SYNC
 			   1: MSIOF_SS1
 			   2: MSIOF_SS2
+			 Hardware limitations related to chip selects:
+			   - Native chip selects are always deasserted in
+			     between transfers that are part of the same
+			     message.  Use cs-gpios to work around this.
+			   - All slaves using native chip selects must use the
+			     same spi-cs-high configuration.  Use cs-gpios to
+			     work around this.
+			   - When using GPIO chip selects, at least one native
+			     chip select must be left unused, as it will be
+			     driven anyway.
 - dmas                 : Must contain a list of two references to DMA
 			 specifiers, one for transmission, and one for
 			 reception.