doc: board/qualcomm: document generic targets

Replace the board specific docs with a generic board.rst file which
documents the build/boot process for the sdm845 and qcs404 boards now
that the only differences are the DTB in use.

At the same time, create a debugging page to document some useful
snippets and tips for working with Qualcomm platforms.

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index 40b03d2..5a4bfbf 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -573,6 +573,7 @@
 S:	Maintained
 F:	arch/arm/mach-snapdragon/
 F:	configs/qcom_defconfig
+F:	doc/board/qualcomm
 F:	drivers/button/button-qcom-pmic.c
 F:	drivers/clk/qcom/
 F:	drivers/gpio/msm_gpio.c