[sw, dif_aon_timer] Introduce DIF AON Timer library

This change introduces public API for the AON Timer peripheral. It
also adds register definitions generation and blank implementation
to make this library compile.

Signed-off-by: Silvestrs Timofejevs <silvestrst@lowrisc.org>
diff --git a/meson.build b/meson.build
index 9a6fd3e..2b12355 100644
--- a/meson.build
+++ b/meson.build
@@ -180,6 +180,7 @@
 # TODO: Considering moving these into hw/ip directories.
 hw_ip_aes_reg_h = gen_hw_hdr.process('hw/ip/aes/data/aes.hjson')
 hw_ip_alert_handler_reg_h = gen_hw_hdr.process('hw/ip/alert_handler/data/alert_handler.hjson')
+hw_ip_aon_timer_reg_h = gen_hw_hdr.process('hw/ip/aon_timer/data/aon_timer.hjson')
 hw_ip_clkgmr_reg_h = gen_hw_hdr.process('hw/top_earlgrey/ip/clkmgr/data/autogen/clkmgr.hjson')
 hw_ip_flash_ctrl_reg_h = gen_hw_hdr.process('hw/top_earlgrey/ip/flash_ctrl/data/autogen/flash_ctrl.hjson')
 hw_ip_gpio_reg_h = gen_hw_hdr.process('hw/ip/gpio/data/gpio.hjson')