Documentation/mainboard: Add emulation/spike-riscv.md

Move the usage instructions from their ad-hoc place in Kconfig.name to
the Documentation directory, and expand them a bit.

Change-Id: Id6c7bbca40a21ecba00cab736af2f2662a985106
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/28874
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Philipp Hug <philipp@hug.cx>

Documentation/mainboard/index.md

@@ -10,6 +10,12 @@ This section contains documentation about coreboot on specific mainboards.
 
 - [CN81XX EVB SFF](cavium/cn8100_sff_evb.md)
 
+## Emulation
+
+The boards in this section are not real mainboards, but emulators.
+
+- [Spike RISC-V emulator](emulation/spike-riscv.md)
+
 ## Foxconn
 
 - [D41S](foxconn/d41s.md)