dt-bindings: connector: Add slow-charger-loop definition

Allows PMIC charger loops which are slow(i.e. cannot meet the
15ms deadline) to still comply to pSnkStby i.e Maximum power
that can be consumed by sink while in Sink Standby state as defined
in 7.4.2 Sink Electrical Parameters of USB Power Delivery Specification
Revision 3.0, Version 1.2.

This patch introduces slow-charger-loop which when set makes
the port request PD_P_SNK_STDBY_MW(2.5W i.e 500mA@5V) upon entering
SNK_DISCOVERY (instead of 3A or the 1.5A during SNK_DISCOVERY) and the
actual currrent limit after RX of PD_CTRL_PSRDY for PD link or during
SNK_READY for non-pd link.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
Link: https://lore.kernel.org/r/20210414142656.63749-3-badhri@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>