of: provide of_machine_read_compatible()

of: provide of_machine_read_compatible()

Provide a helper function allowing users to read the compatible string
of the machine, hiding the access to the root node.

Reviewed-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20260223-soc-of-root-v2-1-b45da45903c8@oss.qualcomm.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>