regulator: max8973: add support to configure ETR from DT

The MAX8973/MAX77621 feature an Enhanced Transient Response(ETR)
circuit that is enabled through software. The enhanced transient
response reduces the voltage droop during large load steps by
temporarily allowing all three phases to fire in unison, slewing
total inductor current faster than would normally be possible if
all three phases continued to operate 120deg out of phase. The
enhanced transient response detector features two selectable
sensitivity settings, which select the output voltage slew rate
during load transients that triggers the ETR circuit. The sensitivity
of the ETR detector is set by the CKADV[1:0] bits in the CONTROL2
register.

Add support to configure the ETR through platform data from DT.
Update the DT binding document accordingly.

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