ARM: meson: merge Kconfig symbol MACH_MESON8B into MACH_MESON8

Currently there are no differences between the MACH_MESON8 and
MACH_MESON8B Kconfig symbols (except the help text). Since both
platforms are very similar (Meson8b being a slightly updated,
cost-reduced version of Meson8 which even shares some peripherals with
Meson8m2) no notable differences are expected in the future either.

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