powerpc: cast bi_memsize to ulong for %ld usage

When exporting the new memsize without reserved PRAM area, the -Wformat
option produces a warning since %ld is used for snprintf and bi_memsize
is phys_size_t.

This patch removes this warning for all PRAM PowerPC boards.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>

arch/powerpc/lib/board.c

@@ -984,7 +984,7 @@ void board_init_r(gd_t *id, ulong dest_addr)
 		pram += (LOGBUFF_LEN + LOGBUFF_OVERHEAD) / 1024;
 #endif
 #endif
-		sprintf(memsz, "%ldk", (bd->bi_memsize / 1024) - pram);
+		sprintf(memsz, "%ldk", (ulong) (bd->bi_memsize / 1024) - pram);
 		setenv("mem", memsz);
 	}
 #endif