ARM: dts: meson: build the Meson8b .dtbs with MACH_MESON8

Currently there are two identical Kconfig options where only differences
are the Kconfig help text and the list of .dtbs that are built:
- MACH_MESON8
- MACH_MESON8B

Build the Meson8b .dtbs when MACH_MESON8 is selected to get rid of the
latter Kconfig symbol later.

Suggested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>