soc: add deprecation warning if using HWMv1 SoCs

While all in-tree SoCs have been ported to HWMv2, Zephyr still supports
out-of-tree SoCs in HWMv1 format, including boards. Add a clear
deprecation message so that users get notified that this is a deprecated
feature to be removed in the future.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>