[PARISC] move defconfig to arch/parisc/configs/

This patch moves the default parisc defconfig to
arch/parisc/configs/generic_defconfig where it belongs and selects it as
the default defconfig through KBUILD_DEFCONFIG.

Signed-off-by: Adrian Bunk <adrian.bunk@movial.fi>
Signed-off-by: Kyle McMartin <kyle@mcmartin.ca>