This is needed with the pl011 driver. Before this change it would default
to a shift of 0, however the hardware places the registers at 4-byte
addresses meaning the value should be 2.
This patch fixes this for the pl011 when configured using the fdt. The
other drivers have a default value of 0 to keep this a no-op.
MFC after: 1 week
uart implementations, and export them using the new linker-set mechanism.
Differential Revision: https://reviews.freebsd.org/D1993
Submitted by: Michal Meloun
