Allow reset vector to be set dynamically

A chip's power-up sequence, or awake-from-sleep sequence, may wish to
set the reset PC based upon dynamic properties, e.g., the settings of
external pins.  Support this by passing the reset vector to the Coreplex.
ExampleTop simply hard-wires the reset vector, as was the case before.

Additionally, allow MTVEC to *not* be reset.  In most cases, including
riscv-tests, pk, and bbl, overriding MTVEC is one of the first things
that the boot sequence does.  So the reset value is superfluous.

src/main/scala/rocketchip/Top.scala

@@ -103,6 +103,7 @@ class ExampleTopBundle(p: Parameters, c: Coreplex) extends BaseTopBundle(p, c)
 class ExampleTopModule[+L <: ExampleTop, +B <: ExampleTopBundle](p: Parameters, l: L, b: Coreplex => B) extends BaseTopModule(p, l, b)
     with PeripheryBootROMModule with PeripheryDebugModule with PeripheryExtInterruptsModule with PeripheryCoreplexLocalInterrupterModule
     with PeripheryMasterMemModule with PeripheryMasterMMIOModule with PeripherySlaveModule
+    with HardwiredResetVector
 
 /** Example Top with TestRAM */
 class ExampleTopWithTestRAM(q: Parameters) extends ExampleTop(q)