regulator: qpnp-amoled: Add set_load() callback for IBB regulator

Add support for set_load() callback for IBB regulator device so that
display driver can call regulator_set_load() to configure dual phase
for selfie flash use case.

For IBB regulator:
 If load current <= 30mA, use single phase
 If load current > 30mA, use dual phase.

Change-Id: I93698a974484f0919893af31f6dfa27ebb30a4e4
Signed-off-by: Shyam Kumar Thella <sthella@codeaurora.org>