clk: armada-37xx: Set DM_FLAG_PRE_RELOC

Setting DM_FLAG_PRE_RELOC for Armada 3720 clock drivers (TBG and
peripheral clocks) makes it possible for serial driver to retrieve clock
rates via clk API.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>