parisc: update kbuild doc. aliases for parisc64

ARCH=parisc64 is now supported for 64-bit parisc builds, so add
this alias to the kbuild.rst documentation.

Fixes: 3dcfb729 ("parisc: Make CONFIG_64BIT available for ARCH=parisc64 only")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-parisc@vger.kernel.org
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: linux-kbuild@vger.kernel.org
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Acked-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Helge Deller <deller@gmx.de>