Add target_data_bits().

Add target_data_bits().

This is used to compute memory block read alignment, and specifically
allows 64-bit targets to ensure that memory block reads are only
requested on 64-bit boundaries.

Signed-off-by: Tim Newsome <tim@sifive.com>
Change-Id: Idb1a27b9fc02c46245556bb0f3d6d94b368c4817
Reviewed-on: http://openocd.zylin.com/6249
Reviewed-by: Marc Schink <dev@zapb.de>
Tested-by: jenkins
Reviewed-by: Jan Matyas <matyas@codasip.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>