ARM: dts: rockchip: Add ramp delay for vdd_cpu in firefly board dts

Adds ramp delay for the vdd_cpu output. It removes warning "ramp_delay
not set" emitted by the function regulator_set_voltage_time_sel() by the
same time, which floods kernel logs.

Signed-off-by: Romain Perier <romain.perier@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>