[sw/example] add SLINK demo program

[stnolting]

This PR adds a demo program for the SLINK module (sw/example/demo_slink; written for the updates from #236).
It provides an interactive UART console to trigger SLINK read/write transactions by hand:

<<< SLINK Access Test Program >>>

This program allows to create SLINK transfers by hand.
Type 'help' to see the help menu.

SLINK_ACCESS:> help
Available commands:
 help     - show this text
 status   - show SLINK HW status
 setup    - configure SLINK module
 read     - read from SLINK channel
 write    - write to SLINK channel
 irq_mode - toggle SLINK IRQ mode
 irq_en   - toggle SLINK IRQ enable
 reset    - reset SLINK module

Configure the SLINK module using 'setup'. Then transfer data using 'read' and 'write'.

SLINK_ACCESS:> setup
Select SLINK access mode:
 n: non-blocking access -> check SLINK status flags before access
 b: blocking access     -> raise an exception on invalid access request
Select: n

SLINK_ACCESS:> status
Hardware configuration
 TX links: 1
 RX links: 1
 TX FIFO:  8 entries
 RX FIFO:  8 entries

SLINK status:
 Link status: 0x00000100
 IRQ config.: 0x00000000

SLINK_ACCESS:> write
Enter TX channel ID (0..0): 0
Enter TX data (8 hex chars): 0xCAFE1234
Writing '0xcafe1234' to TX channel 0...
Write successful.

SLINK_ACCESS:> read
Enter RX channel ID (0..0): 0
Reading from RX channel 0...
RX data: 0xcafe1234

SLINK_ACCESS:>