[SPARC64]: Fix net booting on Ultra5

We were not doing alignment properly when remapping the kernel image.

What we want is a 4MB aligned physical address to map at KERNBASE.
Mistakedly we were 4MB aligning the virtual address where the kernel
initially sits, that's wrong.

Instead, we should PAGE align the virtual address, then 4MB align the
physical address result the prom gives to us.

Signed-off-by: David S. Miller <davem@davemloft.net>