gxbb: clk: Adjust MESON_GATE macro to be shared with meson8b

The macro used gxbb_ prefix for clock definitions. In order
to share the macro between gxbb and meson8b, the prefix must
be moved to gxbb.c.

Signed-off-by: Alexander Müller <serveralex@gmail.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/1472319654-59048-6-git-send-email-serveralex@gmail.com