Update u-boot-rpi to the latest versions of u-boot and firmware files.

The updated boot firmware reportedly supports the new Rpi Zero as well as
existing models. The latest u-boot now runs with caches enabled for faster
booting.

These changes add the standard freebsd API patch to flush caches on device
close. Also, CONFIG_CMD_ELF is removed from our config changes, because it's
now on by default for rpi in 2016.01.

Approved by: sbruno(mentor)
Differential Revision: https://reviews.freebsd.org/D5150