85xx: properly document MPC85xx_PORDEVSR2_SEC_CFG

Commit f7d190b1 corrected the value of MPC85xx_PORDEVSR2_SEC_CFG, but forgot
to add a comment that the correct value disagrees with the 8544 reference
manual.  The changelog for that commit is also wrong, as it says "bit 28"
when it should be "bit 24".

Signed-off-by: Timur Tabi <timur@freescale.com>