sparc64: Fix build warnings in piggyback_64.c

This patch fixes the following build warnings:
arch/sparc/boot/piggyback_64.c: In function 'main':
arch/sparc/boot/piggyback_64.c:44: warning: 'end' may be used uninitialized in this function
arch/sparc/boot/piggyback_64.c:44: warning: 'start' may be used uninitialized in this function

Signed-off-by: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>