These changes attempt to put things in order before the introduction of MIPS
ubldr.

The changes are mostly dealing with removing unnecessary casts from the U-Boot
API (we're passing only pointers, no obvious reason to cast them to uint32_t),
cleaning up some compiler warnings and using the proper printf format
specifiers in order to be able to compile cleanly for both 32-bit and 64-bit
MIPS targets.

Reviewed by: imp
Approved by: adrian (mentor)
Sponsored by: Smartcom - Bulgaria AD
Differential Revision: https://reviews.freebsd.org/D5312