riscv: dts: sifive: "fix" pmic watchdog node name

After converting the pmic watchdog binding to yaml, dtbs_check complains
that the node name doesn't match the binding. "Fix" it.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20220606201343.514391-5-mail@conchuod.ie
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>