rtc: rp5c01: add NVRAM support

The Ricoh RP5C01 RTC contains 26 x 4 bits of NVRAM.  Provide access to it
via a sysfs "nvram" attribute file.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Wan ZongShun <mcuos.com@gmail.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>