hw/timer: Add NPCM7xx Timer device model

hw/timer: Add NPCM7xx Timer device model

The NPCM730 and NPCM750 SoCs have three timer modules each holding five
timers and some shared registers (e.g. interrupt status).

Each timer runs at 25 MHz divided by a prescaler, and counts down from a
configurable initial value to zero. When zero is reached, the interrupt
flag for the timer is set, and the timer is disabled (one-shot mode) or
reloaded from its initial value (periodic mode).

This implementation is sufficient to boot a Linux kernel configured for
NPCM750. Note that the kernel does not seem to actually turn on the
interrupts.

Reviewed-by: Tyrone Ting <kfting@nuvoton.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Alexander Bulekov <alxndr@bu.edu>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Havard Skinnemoen <hskinnemoen@google.com>
Message-id: 20200911052101.2602693-4-hskinnemoen@google.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>