powerpc/boot/dts: Install dts from the right directory

The dts-installed variable is initialised using a wildcard path that
will be expanded relative to the build directory.  Use the existing
variable dtstree to generate an absolute wildcard path that will work
when building in a separate directory.

Reported-by: Gerhard Pircher <gerhard_pircher@gmx.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Tested-by: Gerhard Pircher <gerhard_pircher@gmx.net> [against 2.6.32]
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>