powerpc/prom_init: Replace __initdata with __prombss when applicable

This replaces all occurrences of __initdata for uninitialized
data with a new __prombss

Currently __promdata is defined to be __initdata but we'll
eventually change that.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>