doc: Update support for clang-16 toolchain in README and help text
Also add clang-15 which was missed out in the help text in previous releases.
Signed-off-by: Senthil Kumaran S senthil.kumaran@linaro.org
Also add clang-15 which was missed out in the help text in previous releases.
Signed-off-by: Senthil Kumaran S senthil.kumaran@linaro.org