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>

src/mainboard/emulation/spike-riscv/Kconfig.name

@@ -1,7 +1,3 @@
 config BOARD_EMULATION_SPIKE_RISCV
 	bool "SPIKE riscv"
 	help
-	  To run coreboot in spike:
-	  * run "make" as usual
-	  * util/riscv/make-spike-elf.sh build/coreboot.{rom,elf}
-	  * spike -m1024 build/coreboot.elf