rockchip: rk3399-puma: migrate README to doc/board in rST format

This migrates the plaintext README in
board/theobroma-systems/puma_rk3399 to doc/board/theobroma-systems and
while doing so, update the instructions and rewrite it in rST.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
diff --git a/board/theobroma-systems/puma_rk3399/MAINTAINERS b/board/theobroma-systems/puma_rk3399/MAINTAINERS
index 23701b3..7e84a5b 100644
--- a/board/theobroma-systems/puma_rk3399/MAINTAINERS
+++ b/board/theobroma-systems/puma_rk3399/MAINTAINERS
@@ -4,6 +4,7 @@
 S:	Maintained
 F:	board/theobroma-systems/puma_rk3399
 F:	board/theobroma-systems/common
+F:	doc/board/theobroma-systems
 F:	include/configs/puma_rk3399.h
 F:	arch/arm/dts/rk3399-puma*
 F:	configs/puma-rk3399_defconfig