leds: qpnp-flash-v2: Add operational current property

Add the qcom,current-ma property to specify the operational
current for the flash led. Also limit the minimum flash current
to 25mA since currents below this value cause unstable flash
operation.

CRs-Fixed: 1033071
Change-Id: Ia133b6c0cf0c21484f61631f04cba0d1112c9d48
Signed-off-by: Devesh Jhunjhunwala <deveshj@codeaurora.org>