hw/timer: Add SiFive PWM support

This is the initial commit of the SiFive PWM timer. This is used by
guest software as a timer and is included in the SiFive FU540 SoC.

Signed-off-by: Justin Restivo <jrestivo@draper.com>
Signed-off-by: Alexandra Clifford <aclifford@draper.com>
Signed-off-by: Amanda Strnad <astrnad@draper.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Message-id: 9f70a210acbfaf0e1ea6ad311ab892ac69134d8b.1631159656.git.alistair.francis@wdc.com