refactor(fvp): fdts: merge motherboard .dtsi files

refactor(fvp): fdts: merge motherboard .dtsi files

For no real reason we were shipping two separate DT include files for the
base FVP motherboard peripherals, one for aarch32, one for aarch64.
There is no difference in the hardware description when using a
different instruction set, and the diff between the two files was about
a missing interrupt map for the 64-bit DT files.

Consolidate the situation by just using a single motherboard .dtsi file,
which relies on an interrupt map by the including files.
Provide that map in the two files where it was missing before, and
change the filenames to let all users include the same file now.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Change-Id: I19b77ecc8da9b4bfbd61d02f910b9ab05dbf92e9