nios2: divide nios2-io.h into each specific drivers and remove it

The nios2-io.h defines hardware registers and bits of several FPGA
IP cores. It could be divided in to the specific drivers, including
altera timer, altera sysid, altera uart and altera jtag uart. The
altera pio and altera spi drivers use their own hardware definitions.
The removal of nios2-io.h will help modularity and maintenance.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>