mfd: ab8500: Introduce DEFINE_SHOW_ATTRIBUTE() macro

This macro deduplicates a lot of similar code in the ab8500-debugfs.c module.
Targeting to be moved to seq_file.h eventually.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>