imx: mx7: Add comment to describe OTP TESTER registers

The tester registers provide a unique chip-level identifier which
get_board_serial() returns in a "struct tag_serialnr".

This patch documents the properties of the registers; in summary.

31:0 OCOTP_TESTER0 (most significant)
- FSL-wide unique, encoded LOT ID STD II/SJC CHALLENGE/ Unique ID

OCOTP_TESTER1 (least significant)
31:24
- The X-coordinate of the die location on the wafer/SJC CHALLENGE/ Unique
  ID
23:16
- The Y-coordinate of the die location on the wafer/SJC CHALLENGE/ Unique
  ID
15:11
- The wafer number of the wafer on which the device was fabricated/SJC
  CHALLENGE/ Unique ID
10:0
- FSL-wide unique, encoded LOT ID STD II/SJC CHALLENGE/ Unique ID

The 64 bits of data generate a unique serial number per-chip.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>