doc: board/qualcomm: describe phone support and bringup

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>