Fix excise_initrd_region() to support 32- and 64-bit initrd params.

Changed excise_initrd_region to support both 32- and 64-bit
values for linux,initrd-start and linux,initrd-end.

This fixes the boot problem on some machines after rS334485.

Submitted by: Luis Pires <lffpires@ruabrasil.org>
Reviewed by: jhibbits, leitao
Approved by: jhibbits (mentor)
Differential Revision: https://reviews.freebsd.org/D15667