[bazel] Autogenerate register headers from hjson
1. Create a rule to execute `regtool` under bazel.
2. Add rules for `top_earlgrey.h` and friends.
3. Instatiate the rule for each of the various IP blocks, resulting in
the following set of labels which may be used as deps in other rules:
```
//hw/ip/aes/data:aes_regs
//hw/ip/aon_timer/data:aon_timer_regs
//hw/ip/csrng/data:csrng_regs
//hw/ip/edn/data:edn_regs
//hw/ip/entropy_src/data:entropy_src_regs
//hw/ip/gpio/data:gpio_regs
//hw/ip/hmac/data:hmac_regs
//hw/ip/i2c/data:i2c_regs
//hw/ip/keymgr/data:keymgr_regs
//hw/ip/kmac/data:kmac_regs
//hw/ip/lc_ctrl/data:lc_ctrl_regs
//hw/ip/otbn/data:otbn_regs
//hw/ip/otp_ctrl/data:otp_ctrl_regs
//hw/ip/rv_timer/data:rv_timer_regs
//hw/ip/spi_device/data:spi_device_regs
//hw/ip/sram_ctrl/data:sram_ctrl_regs
//hw/ip/uart/data:uart_regs
//hw/ip/usbdev/data:usbdev_regs
//hw/top_earlgrey/ip/alert_handler/data/autogen:alert_handler_regs
//hw/top_earlgrey/ip/clkmgr/data/autogen:clkmgr_regs
//hw/top_earlgrey/ip/flash_ctrl/data/autogen:flash_ctrl_regs
//hw/top_earlgrey/ip/pinmux/data/autogen:pinmux_regs
//hw/top_earlgrey/ip/pwrmgr/data/autogen:pwrmgr_regs
//hw/top_earlgrey/ip/rstmgr/data/autogen:rstmgr_regs
//hw/top_earlgrey/ip/rv_plic/data/autogen:rv_plic_regs
```
Signed-off-by: Chris Frantz <cfrantz@google.com>
diff --git a/hw/ip/gpio/BUILD b/hw/ip/gpio/BUILD
new file mode 100644
index 0000000..fd60e61
--- /dev/null
+++ b/hw/ip/gpio/BUILD
@@ -0,0 +1,4 @@
+# Copyright lowRISC contributors.
+# Licensed under the Apache License, Version 2.0, see LICENSE for details.
+# SPDX-License-Identifier: Apache-2.0
+
