dm: allow limiting pre-reloc markings to spl or tpl

Right now the u-boot,dm-pre-reloc flag will make each marked node
always appear in both spl and tpl. But systems needing an additional
tpl might have special constraints for each, like the spl needing to
be very tiny.

So introduce two additional flags to mark nodes for only spl or tpl
environments and introduce a function dm_fdt_pre_reloc to automate
the necessary checks in code instances checking for pre-relocation
flags.

The behaviour of the original flag stays untouched and still marks
a node for both spl and tpl.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Kever Yang <kever.yang@rock-chips.com>