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.be
Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>