Add keep_alive() to system bus read loop.

Without it the repeat read test wasn't passing any longer. This is
because 7dd323b26d reduced the remote
bitbang performance. I've notified the mailing list about this.

Change-Id: Ie71592792202423aec89fa889b9e3d2a60a3c25f
Signed-off-by: Tim Newsome <tim@sifive.com>

src/target/riscv/riscv-013.c

@@ -2745,6 +2745,7 @@ static int read_memory_bus_v1(struct target *target, target_addr_t address,
 				}
 				next_read = address + i * size + j * 4;
 			}
+			keep_alive();
 		}
 
 		uint32_t sbcs_read = 0;