Fix casts between 64-bit physical addresses and pointers in EFI.

Compiling FreeBSD/i386 with modern GCC triggers warnings for various
places that convert 64-bit EFI_ADDRs to pointers and vice versa.

• Cast pointers to uintptr_t rather than to uint64_t when assigning to a 64-bit integer.
• Cast 64-bit integers to uintptr_t before a cast to a pointer.

Reviewed by: kevans
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D16586