zephyr: charger: Make the charge-ramp method as optional
zephyr: charger: Make the charge-ramp method as optional

This CL Makes charge-ramp method optional to support disabling
ramp-method if project doesn't requires that.

BUG=b:218684235
BRANCH=none
TEST=Build Successful.

Signed-off-by: Rajesh Kumar <ef973afabdaa06670be7215ef6ba2acd6d71d568@intel.com>
Change-Id: I023a4cd0a41247affa90c90d33577a82ad3b54f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3486196
Reviewed-by: Vijay P Hiremath <006c712879a3af08bfdb5c2b8491a8b2d5fd73cd@intel.com>
Reviewed-by: Keith Short <f6d578f97033c25f1206cb57d52f358974c82954@chromium.org>

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