dtc: Remove unused variable in flat_read_mem_reserve

The *p variable is declared and used to save inb->ptr, however p is
later never used.  This has been the case since commit 6c0f3676 and can
lead to build failures with -Werror=unused-but-set-variable:

	flattree.c: In function 'flat_read_mem_reserve':
	flattree.c:700:14: error: variable 'p' set but not used [-Werror=unused-but-set-variable]
	cc1: all warnings being treated as errors
	make: *** [flattree.o] Error 1

Remove the variable.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>