spl: dm: Make it possible for the SPL to pick its own DTB from a FIT

u-boot can be embedded within a FIT image with multiple DTBs. It then
selects at run-time  which one is best suited for the platform.
Use the same principle here for the SPL: put the DTBs in a FIT image,
compress it (LZO, GZIP, or no compression) and append it at the end of the
SPL.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
[trini: Move default y of SPL_MULTI_DTB_FIT_DYN_ALLOC to it being the
default choice if SYS_MALLOC_F, drop spl.h include from lib/fdtdec.c
it's unused.]
Signed-off-by Tom Rini <trini@konsulko.com>