leds: qpnp-wled: Add support to configure VREF_CONTROL for pmicobalt

WLED module in pmicobalt has different VREF_CONTROL settings with current
WLED module which is existing in pmi8994/pmi8952, including min voltage,
max voltage, step size and default voltage. Add support for it.

CRs-Fixed: 1081738
Change-Id: Ibaa555cec8cfd296125af3e6277df5331f5c438b
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>