Unaligned addresses with axi_read_slave

[crdavis12]

Fixing how axi_read_slave handles unaligned addresses.

Let's say your memory looks like this:
byte 0 = 0x00
byte 1 = 0x01
byte 2 = 0x02
byte 3 = 0x03
byte 4 = 0x04
byte 5 = 0x05

AXI4 read request looks like this:
bus width = 4 bytes
ARADR = 2
ARLEN = 0 (1 word)

On RDATA I would expect to see 0x03020100 or maybe 0x0302---- since the lower 2 bytes aren't valid.

Instead you get 0x03020504 for the first transfer, 0x07060908 for the second, etc. This change fixes the rotated data.

Mentioned in #851 but not exactly the same issue.

[crdavis12]

TODO: Need to update tests

[crdavis12]

Any word on when this might be able to be merged? I heard something about breaking verification components out into a different repo -- planning to wait for that first?