usb: typec: tcpm: Refactor logic for new-source-frs-typec-current

New source's current capability is now defined as string based
device tree property through new-source-frs-typec-current.
Refactor tcpm code to parse new-source-frs-typec-current and
infer local port's new source current capability during frs.

Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
Link: https://lore.kernel.org/r/20201029063138.1429760-4-badhri@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>