meson: clk: Move register definitions to meson8b.h

Move the register definitions into a separate header file
to reflect the gxbb implementation.

Signed-off-by: Alexander Müller <serveralex@gmail.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Link: lkml.kernel.org/r/1472319654-59048-3-git-send-email-serveralex@gmail.com