Move to using a refined pr_fmt to avoid having to manually
prefix every message line with 'ts78xx'.

Changelog:
 v2: moved pr_fmt define ahead of includes as suggested by
	Hartley Sweeten to avoid use of leading undef
 v1: initial release
Signed-off-by: NAlexander Clouter <alex@digriz.org.uk>
Signed-off-by: NAndrew Lunn <andrew@lunn.ch>