kbuild: deb-pkg: add debarch for ARCH=loongarch64

Fix follow warning when 'make ARCH=loongarch64 bindeb-pkg':

  ** ** **  WARNING  ** ** **

  Your architecture doesn't have its equivalent
  Debian userspace architecture defined!
  Falling back to the current host architecture (loong64).
  Please add support for loongarch64 to ./scripts/package/mkdebian ...

Reported-by: Shiwei Liu <liushiwei@anheng.com.cn>
Signed-off-by: WangYuli <wangyuli@uniontech.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>