bits: add comments and newlines to #if, #else and #endif directives

This is a preparation for the upcoming GENMASK_U*() and BIT_U*()
changes. After introducing those new macros, there will be a lot of
scrolling between the #if, #else and #endif.

Add a comment to the #else and #endif preprocessor macros to help keep
track of which context we are in. Also, add new lines to better
visually separate the non-asm and asm sections.

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>