mtd: bcm47xxpart: support layouts with multiple TRX partitions

Some devices may have an extra TRX partition used as failsafe one. If
we detect such partition we should set a proper name for it and don't
parse it.

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