asm: add Hostboot rom_entry.S
asm: add Hostboot rom_entry.S

This adds the Hostboot rom_entry.S used to jump into the verification code copied to RAM.
https://github.com/open-power/hostboot/blob/master/src/usr/secureboot/base/rom_entry.S

Signed-off-by: Claudio Carvalho <e431f4b4a17082357e3d270b337bd485cc81832b@linux.vnet.ibm.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

mikey/skiboot,legoater/skiboot,legoater/skiboot,qemu/skiboot,legoater/skiboot,shenki/skiboot,open-power/skiboot,qemu/skiboot,stewart-ibm/skiboot,open-power/skiboot,qemu/skiboot,qemu/skiboot,shenki/skiboot,mikey/skiboot,shenki/skiboot,mikey/skiboot,open-power/skiboot,shenki/skiboot,stewart-ibm/skiboot,qemu/skiboot,legoater/skiboot,legoater/skiboot,open-power/skiboot,stewart-ibm/skiboot,open-power/skiboot,shenki/skiboot