rtc: m48t59: Use platform_data struct for year offset value

Instead of hard-coded values and ifdefs, store the year offset in the
platform_data struct.

Tested-by: Daniel Palmer <daniel@0x0f.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
Tested-by: Andreas Larsson <andreas@gaisler.com>
Acked-by: Andreas Larsson <andreas@gaisler.com>
Link: https://lore.kernel.org/r/665c3526184a8d0c4a6373297d8e7d9a12591d8b.1731450735.git.fthain@linux-m68k.org


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>