power_supply: Add TYPEC_SRC_RP property

Add property POWER_SUPPLY_PROP_TYPEC_SRC_RP to return the Rp
currently configured for the charger while in Type-C Source
mode.

Also add enum listing the set of all valid Rp values.

CRs-Fixed: 2283665
Change-Id: I234a9095192ccfefbfc3c38cf6c07861bd7e2316
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>