usb: pd: support PD 3.0 extended messages with chunking

Add support for receiving extended messages. Since the underlying
PD PHY only supports a fixed 30 byte payload, longer messages up
to 260 bytes must use chunking, which requires the receiver to request
the next chunk until the entire payload is delivered.

When receiving a chunk, call a helper from phy_msg_received() to stage
the received bytes into a long rx_msg and queue a request for the
next chunk. Refactor struct rx_msg to use a flexible array member to
accommodate the larger potential sizes.

Finally, add new enum definitions for the new PD 3.0 message types.

Change-Id: I962ad3a1f00db9c16133301d185bf123af2a3224
Signed-off-by: Jack Pham <jackp@codeaurora.org>