pci: pcie_dw_rockchip: Use regulator_set_enable_if_allowed

The vpcie3v3 regulator is typically a fixed regulator controlled using
gpio. Change to use enable and disable calls on the regulator instead
of trying to set a voltage value.

Also remove the delay to match linux driver, for a fixed regulator the
startup-delay-us prop can be used in case a startup delay is needed.
Limited testing on ROCK 3A, ROCK 5B, Quartz64, Odroid-M1 has shown that
this delay was not needed.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>