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>

arch/arm/mach-imx/mach-imx7ulp.c

@@ -37,6 +37,7 @@ static void __init imx7ulp_set_revision(void)
 	 * bit[31:28] of JTAG_ID register defines revision as below from B0:
 	 * 0001        B0
 	 * 0010        B1
+	 * 0011        B2
 	 */
 	switch (revision >> 28) {
 	case 1: