[POWERPC] Fix loop with unsigned long counter variable

This fixes a possible infinite loop when the unsigned long counter "i"
is used in lmb_add_region() in the following for loop:

for (i = rgn->cnt-1; i >= 0; i--)

by making the loop counter "i" be signed.

Signed-off-by: Manish Ahuja <ahuja@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>