boards: riscv: opentitan_earlgrey: Add the OpenTitan AON Timer

Adds the AON Timer device in the OpenTitan Earlgrey device tree.
Adds overlay files to enable the watchdog and set the alias to
`watchdog0`.

Adds the AON timer (watchdog part) to the supported features section
of the OpenTitan documentation.

Signed-off-by: Tyler Ng <tkng@rivosinc.com>
5 files changed