parisc: rename default_defconfig to defconfig

'default_defconfig' is an awkward name since 'defconfig' is the default.
Let's simply say 'defconfig' like other architectures. You can drop the
KBUILD_DEFCONFIG define by following the standard naming.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Helge Deller <deller@gmx.de>