ARM: dts: socfpga: Factor out U-Boot specifics from A10 handoff files

Pull out the u-boot,dm-pre-reloc from socfpga_arria10_socdk_sdmmc_handoff.dtsi
into separate dtsi header file to make it easier to patch in custom handoff
dtsi files, without having to manually add the U-Boot bits. Shuffle the include
clauses in the A10 DT files to make it obvious what gets included where without
having to follow confusing long chain of includes, i.e. board DT file includes
everything it needs.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <chin.liang.see@intel.com>
Cc: Dinh Nguyen <dinguyen@kernel.org>
Cc: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Cc: Tien Fong Chee <tien.fong.chee@intel.com>