docs/manual: update external toolchain examples

Replace ADI by Analog Devices, which is clearer for most people, and
remove the Xilinx external toolchains since we have deprecated them.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>