serial: move nios2-yanu.h into opencores_yanu driver

The nios2-yanu.h contains hardware registers and bits of
opencores yanu. As there is no other user of this header
, it should be moved into the driver.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
CC: Renato Andreola <renato.andreola@imagos.it>