doc: board/qualcomm: describe phone support and bringup

Add some documentation which attempts to describe Qualcomm smartphone
support with the qcom-phone.config fragment, as well as a high level
debugging guide for diagnosing U-Boot issues when UART and framebuffer
are unavailable.

Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Danila Tikhonov <danila@jiaxyga.com> # google-sunfish
Tested-by: Jens Reidel <adrian@mainlining.org> # xiaomi-davinci
Link: https://lore.kernel.org/r/20250331-qcom-phones-v4-4-f52e57d3b8c6@linaro.org
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
diff --git a/doc/board/qualcomm/board.rst b/doc/board/qualcomm/board.rst
index 4d79320..003d59a 100644
--- a/doc/board/qualcomm/board.rst
+++ b/doc/board/qualcomm/board.rst
@@ -90,6 +90,11 @@
 	make CROSS_COMPILE=aarch64-linux-gnu- O=.output dragonboard410c_defconfig
 	make O=.output -j$(nproc)
 
+Or for smartphones::
+
+	make CROSS_COMPILE=aarch64-linux-gnu- O=.output qcom_defconfig qcom-phone.config
+	make O=.output -j$(nproc)
+
 - gzip u-boot::
 
 	gzip u-boot-nodtb.bin