parisc: add vdso linker script to 'targets' instead of extra-y

The vdso linker script is preprocessed on demand.
Adding it to 'targets' is enough to include the .cmd file.

This commit applies the previous change to parisc, which added the
vdso support after commit 887af6d7 ("arch: vdso: add vdso linker
script to 'targets' instead of extra-y").

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Helge Deller <deller@gmx.de>
Signed-off-by: Helge Deller <deller@gmx.de>