ARM: imx: imx7ulp: Add a comment explaining the B2 silicon version

To help the users to understand the meaning of bits 31-28 of the
JTAG_ID_REG register, add a comment explaining the value that is
expected from a i.MX7ULP rev B2.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>