powerpc: Delete old orphaned PrPMC 280/2800 DTS and boot file.

In commit 3c8464a ("powerpc:
Delete old PrPMC 280/2800 support") we got rid of most of the C
code, and the Makefile/Kconfig hooks, but it seems I left the
platform's DTS file orphaned in the tree as well as the boot code.
Here we get rid of them both.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>