sun4u: switch m48t59 NVRAM to MMIO access

sun4u: switch m48t59 NVRAM to MMIO access

Real sun4u systems memory-map the NVRAM on the (ISA) ebus, so switch over to
MMIO from ioport access whilst setting the base year to 1968 as used by Sun
systems. This allows all SPARC64 OSs included in my tests to correctly detect
the NVRAM IC and read the hardware clock correctly upon boot.

Note that this also requires a corresponding OpenBIOS update to r1330 in order
to switch the SPARC64 NVRAM accessors over from ioport to MMIO.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>