atmega328p: Add patch for 8-bit asynchronous timer

Signed-off-by: Rahix <rahix@rahix.de>

atmega32u4,atmega328p: Add patch for 16-bit timers

Signed-off-by: Rahix <rahix@rahix.de>

patch: Restructure timer patches

Redesign timer patch file structure for better modularity.  Now,
`timer/<chip>.yaml` defines all timer peripherals for a chip and uses
peripheral-level includes for patching the actual peripheral.  The
actual peripheral patches live in `timer/dev/<type>.yaml`.

The intermediate `<chip>.yaml` file is needed because peripheral-level
includes are not allowed in top-level files.

Signed-off-by: Rahix <rahix@rahix.de>