Support epoch of 1980 in RTC emulation for MIPS Magnum

Support epoch of 1980 in RTC emulation for MIPS Magnum

On the MIPS Magnum, the time that is held in the RTC's NVRAM should be
relative to midnight on 1980-01-01.  This patch adds an extra parameter
to rtc_init(), allowing different epochs to be used.  For the Magnum,
1980 is specified, and for all other machines, 2000 is specified.

I've not modified the handling of the century byte, as with an epoch of
1980 and a year of 2009, one could argue that it should hold either
0, 1, 19 or 20.  NT 3.50 on MIPS does not read the century byte.

Signed-off-by: Stuart Brady <stuart.brady@gmail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6429 c046a42c-6fe2-441c-8c8c-71466251a162