driver: secure: use consttime_memequal for memory comparison

Do  not  use memcmp() to compare security critical data, such as
cryptographic secrets, because the required CPU time depends on the
number of equal bytes.
Instead, a function that performs comparisons in constant time is required.
Warning: consttime_memequal returns 0 if data are NOT equal, and 1 if they are
equal.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Nicolas Ferre <nicolas.ferre@microchip.com>