riscv: dts: Add DT support for SiFive FU540 PWM driver

Add the PWM DT node in SiFive FU540 soc-specific DT file.
Enable the PWM nodes in HiFive Unleashed board-specific DT file.

Signed-off-by: Yash Shah <yash.shah@sifive.com>
Cc: Palmer Dabbelt <palmer@sifive.com>
[paul.walmsley@sifive.com: added chip-specific compatible string;
 dropped reg-names string from pwm1]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>