parisc: Zero-initialize newly alloced memblock

Commit 4fe9e1d9 ("parisc: Drop bootmem and switch to memblock")
switched to the memblock allocator, but missed to zero-initialize the
newly allocated memblocks. This lead to crashes on some machines like
the rp3410.

Fixes: 4fe9e1d9

 ("parisc: Drop bootmem and switch to memblock")
Signed-off-by: Helge Deller <deller@gmx.de>