acpidump: Move to acpi.rsdp

Switch from the long obsolete hint.0.acpi.rsdp to acpi.rsdp to get the
root of the ACPI tables.

MFC After: 1 week
Sponsored by: Netflix
Reviewed by: andrew, markj
Differential Revision: https://reviews.freebsd.org/D47079

(cherry picked from commit 5d3fb72667e5794bf16756e00a87338f4acb7f90)