riscv: parse memory regions from EFI map header

Using the newly isolated efi_map interface (subr_efi_map.c).

The primary goal is a greater adherence to the EFI firmware's view of
memory, and protection of its reserved regions. This is a better source
of truth than the device tree.

For example, regions marked as RuntimeServicesData must be excluded from
general memory allocations for EFI runtime services to eventually be
enabled on the platform (planned but not currently in progress).

The secondary impact is when loading a new/custom device tree via
loader(8); use of the EFI map ensures that reservations made by firmware
(OpenSBI, u-boot) are maintained.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49131