Switch to upstream riscv-pk

ucb-bar/chipyard#625 and firesim/firesim#598 obviate the FireSim patches
to the UART driver in bbl, as the UART divisor is now reset to the
proper value without software intervention.

.gitmodules

@@ -3,7 +3,7 @@
 	url = https://github.com/firesim/linux.git
 [submodule "riscv-pk"]
 	path = boards/default/firmware/riscv-pk
-	url = https://github.com/firesim/riscv-pk.git
+	url = https://github.com/riscv/riscv-pk.git
 [submodule "opensbi"]
 	path = boards/default/firmware/opensbi
 	url = https://github.com/riscv/opensbi.git