Update CFU example: use XTEA as "real world" demo application

[stnolting]

This PR is a rework of the Custom Functions Unit (CFU) that replaces the default example application by a "real world" application implementing the royalty-free Extended Tiny Encryption Algorithm (XTEA) as dedicated ISA extension implemented as CFU-based custom RISC-V instructions.

The according demo program computes the encryption and decryption of random data using a 1.) software-only reference and a 2.) hardware-accelerated version using 5 custom XTEA instructions:

neorv32/sw/example/demo_cfu$ make USER_FLAGS+=-DUART0_SIM_MODE clean_all sim
[...]

<<< NEORV32 Custom Functions Unit (CFU) - Custom Instructions Example >>>

[NOTE] This program assumes the default CFU hardware module that
       implements the Extended Tiny Encryption Algorithm (XTEA).

XTEA key: 0x207230ba1ffba710c45271efdd01768a

XTEA SW encryption (40 rounds, 64 words)...
XTEA HW encryption (40 rounds, 64 words)...
Comparing results... OK

XTEA SW decryption (40 rounds, 64 words)...
XTEA HW decryption (40 rounds, 64 words)...
Comparing results... OK

Execution benchmarking:
ENC SW = 51839 cycles
ENC HW = 12852 cycles
DEC SW = 72889 cycles
DEC HW = 12979 cycles

CFU demo program completed.