MIPS: pistachio: Determine SoC revision during boot

Now that there are different revisions of the Pistachio SoC
in circulation, add this information to the boot log to make
it easier for users to determine which hardware they have.

Signed-off-by: James Hartley <james.hartley@imgtec.com>
Signed-off-by: Ionela Voinescu <ionela.voinescu@imgtec.com>
Cc: Andrew Bresticker <abrestic@chromium.org>
Cc: Jonas Gorski <jogo@openwrt.org>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/13130/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>