ARM: meson: add meson8 support

Add a MACH_MESON8 symbol and add "amlogic,meson8" to the list of
compatible strings for the Meson DT machine to support devices based
on the Meson8 family of SoCs.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Carlo Caione <carlo@caione.org>