rtc-ep93xx.c: cleanup probe/remove routines

Fix issue with rtc device not getting unregistered in probe error path.

Use the devres managed resource functions in the probe routine to cleanup
the error path.

Use sysfs_{create/remove}_group to add/remove the sysfs files.

Reduces the text size by 132 bytes, increases data by 12 bytes:
    text	   data	    bss	    dec	    hex	filename
-    937	    124	      0	   1061	    425	rtc-ep93xx.o
+    805	    136	      0	    941	    3ad	rtc-ep93xx.o

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Cc: Paul Gortmaker <p_gortmaker@yahoo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>