sparc: Update PROM initialization code for generic board

Fixed the prom_relocate() function in start.S file by reserving memory in
the board_init_f sequence and saving the offset to the __prom_start_reloc
variable. This value is used as the destination when relocating the PROM.

Add the prom_init() function to the end of the board_init_r sequence.

Signed-off-by: Francois Retief <fgretief@spaceteq.co.za>