Documentation: kernel-parameters: Add earlyprintk=bios on SH

Document the use of the "earlyprintk=bios[,keep]" kernel parameter
option on SuperH systems with a SuperH standard BIOS.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/febc920964f2f0919d21775132e84c5cc270177e.1697708489.git.geert+renesas@glider.beSigned-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>

Documentation/admin-guide/kernel-parameters.txt

@@ -1331,6 +1331,7 @@
 			earlyprintk=dbgp[debugController#]
 			earlyprintk=pciserial[,force],bus:device.function[,baudrate]
 			earlyprintk=xdbc[xhciController#]
+			earlyprintk=bios
 
 			earlyprintk is useful when the kernel crashes before
 			the normal console is initialized. It is not enabled by
@@ -1361,6 +1362,8 @@
 
 			The sclp output can only be used on s390.
 
+			The bios output can only be used on SuperH.
+
 			The optional "force" to "pciserial" enables use of a
 			PCI device even when its classcode is not of the
 			UART class.