microblaze: Remove empty headers

There is no reason to keep empty headers if asm-generic one can be used
instead. That's why remove empty file and use asm-generic equivalent.
cputable.h is completely unused that's why remove it.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Stefan Asserhall <stefan.asserhall@xilinx.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>