Minimal changes for MPR to build on architectures with physical addresses larger than virtual

Summary:
Some architectures use large (36-bit) physical addresses, with smaller
virtual addresses. Casting between vm_paddr_t (or bus_addr_t) and void * is
considered illegal, so cast through uintptr_t. No functional change on existing
platforms.

Reviewed By: scottl
Differential Revision: https://reviews.freebsd.org/D14042