mtd: bcm47xxpart: move TRX parsing code to separated function

This change simplifies main parsing loop logic a bit. In future it may
be useful for moving TRX support to separated module / parser (if we
implement support for them at some point).
Finally parsing TRX at the end puts us in a better position as we have
better flash layout knowledge. It may be useful e.g. if it appears there
is more than 1 TRX partition.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>