meta-ampere: mtmitchell: leds: change to json configuration

phosphor-led-manager changed to use json configuration by default.
However, Mt.Mitchell still uses yaml configuration which consequently
fail to parse for platform LED configuration.
This commit changes configuration from yaml to json to adapt upstream
codes.

Tested:
1. Check if all LED Groups are created, the same as using yaml file
root@mtmitchell-dcscm:~# busctl tree xyz.openbmc_project.LED.GroupManager
`- /xyz
  `- /xyz/openbmc_project
    `- /xyz/openbmc_project/led
      `- /xyz/openbmc_project/led/groups
        |- /xyz/openbmc_project/led/groups/bmc_booted
        |- /xyz/openbmc_project/led/groups/power_on
        |- /xyz/openbmc_project/led/groups/enclosure_identify
        |- /xyz/openbmc_project/led/groups/enclosure_identify_blink
        `- /xyz/openbmc_project/led/groups/systemFault
2. Check if no error message in journal log

Signed-off-by: Thang Q. Nguyen <thang@os.amperecomputing.com>
Change-Id: Ibdc2a566d89d8d661f299aff84539e471061090a