Print more detailed parse error messages

The following errors usually resulted in the same 'Unable to parse link'
message:

- one of the given entities does not exist
- one of the pads of a given entity does not exist
- No link exists between given pads
- syntax error in link description

Add more detailed error messages to give the user a clue what is going wrong.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Print parser position on error

Most parser functions take a **endp argument to indicate the caller
where parsing has stopped. This is currently only used after parsing
something successfully. This patch sets **endp to the erroneous
position in the error case and prints its position after an error
has occured.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
[Remove endp NULL checks spread around the code]
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>