leds: qpnp-flash-v2: Add support to control flash ramp time

Add device tree properties "qcom,ramp-up-step" and
"qcom,ramp-down-step", to configure the time steps for
flash ramp up and ramp down.

Change-Id: I80ff9394ab4af29398522a1abbaba991b67db918
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>