loader: Instrument EFI GetRNG with TSLOG

On some systems, the EFI GetRNG is slow. Make it show up in flamecharts.

MFC after: 1 week
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D46631

(cherry picked from commit b58fe4eb29fbd3236df0f7fdfad4393d2536dbb5)