feat: add an I2C controller for the MMC mailbox

This was tested successfully on a revision A board with fmc1z7io_example_design:

root@mskamcz7io-0002:~# mmcinfo
MMC information
---------------
App version     : 2.1
Lib version     : 2.5
CPLD board ver. : 1.1
CPLD lib ver.   : 1.0
STAMP revision  : Rev. A
AMC slot        : 7
IPMB addr       : 0x7e
Board name      : DAMC-FMC1Z7IO
IANA Vendor ID  : 0x053f
IANA Product ID : 0x2710
Uptime          : 34 days, 19 hours, 37 minutes, 14 seconds

MMC sensors
-----------
STAMP Temp   : 33.3125
AMC MP 3V3   : 3.28507
AMC PP 12V   : 12.8155
I_RTM MP 3V3 : 0
I_RTM PP 12V : 0
CPLD Done    : 1
RTM MP 3V3 P : 0
RTM PP 12V P : 0
RTM Fault    : 0
RTM Temp.1   : nan
PGood_A      : 1
PGood_B      : 1
FPGA Init    : 1
FPGA Done    : 1
Inlet Temp   : 44.125
Z7 IC Temp   : 41.625
IMON_AVCC    : 0.563321
IMON_AVTT    : 0.425299
LTM4644_F Te : 54.8388
LTM4644_R Te : 58.453
FPGA_1V0     : 0.981079
FPGA_1V8     : 1.79956
FPGA_3V3     : 3.24048
FMC_Vadj     : 1.80042
VCC_5V       : 4.94666

FRU 0 description
-----------------
UID           : 801F1234EA7D
Manufacturer  : DESY
Product name  : DAMC-FMC1Z7IO
Part number   : 0000
Serial number : 0002
Version       : 0000

FRU 0 status
-----------------
Flags         : +Present +Compatible +Powered -Failure
Temperature 1 : 44.12 C
Temperature 2 : 41.62 C
Temperature 3 : 58.25 C
Temperature 4 : 57.54 C

FRU 1 not present
-----------------

FRU 2 not present
-----------------

FRU 3 not present
-----------------

FPGA Ctrl: -Shdn -PCIeReset