plat/st: do not rely on tainted value for dt property length
plat/st: do not rely on tainted value for dt property length

To compare the "okay" string of a property, strncmp is used but with the
length given by fdt_getprop. This len value is reported as tainted by
Coverity [1]. We just can use strlen("okay") which is a known value
to compare the 2 strings.

 [1] https://scan4.coverity.com/reports.htm#v51972/p11439/fileInstanceId=96515154&defectInstanceId=14219121&mergedDefectId=342997

Signed-off-by: Yann Gautier <e753f6969c288905606e3c07464886d4f1fc1be0@foss.st.com>
Change-Id: Ic8fb6ccf3126a37df615e433eb028861812015da

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware