regulator: max8973: set startup time for rail based on BIAS control

In MAX8973, BIAS enable control bit is used for changing the startup time
for voltage output. The startup delay is 240us (typ) when the BIASEN
bit is set to 0. The startup delay is reduced to 20us (typ) when the
BIASEN bit is set to 1.

Pass the enable_time through regulator descriptor based on this flag.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@kernel.org>