ARM: berlin: add MACH_BERLIN_BG2Q symbol

Now that we start supporting the Marvell Berlin BG2Q, add a symbol allowing to
differentiate that SoC from the other SoCs of the Berlin family.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>