charger/rt946x: Support software ramping API.
charger/rt946x: Support software ramping API.

Add usb_charger_ramp_allowed() and usb_Charger_ramp_max() API.
Also, targets DCP ramping voltage to 2000mA(charger support
maximum is 3500mA), a conservative value to prevent a bad charger.

TEST=With CL:1610385, plug a DCP 5V/2.1A charger and see it ramps
     and drains up to 1.75A, rather than original 1.34A.
BUG=b:131284131
BRANCH=None

Change-Id: I1e87badbca8991862186fc37227d203cfb54bce3
Signed-off-by: Yilun Lin <2f2733c7206ea2b76bd2db1905cc5e8a75841c48@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/1633930
Commit-Queue: Yilun Lin <2f2733c7206ea2b76bd2db1905cc5e8a75841c48@chromium.org>
Tested-by: Yilun Lin <2f2733c7206ea2b76bd2db1905cc5e8a75841c48@chromium.org>
Reviewed-by: Daisuke Nojiri <fd5f93af191bf7e8f73ea71cc3c0f66b41b1dd49@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec