mx25: Clean up lowlevel_init

Clean up mx25 lowlevel_init:
 - Add comments.
 - Do not use write32 repeatedly with the same value in order not to increase
   code size.
 - Make register values configurable.
 - Use macro parameters with default values instead of literal constants.
 - Use defined macros instead of duplicating code.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: John Rigby <jcrigby@gmail.com>
Cc: Matthias Weisser <weisserm@arcor.de>