driver: secure: move keys into static arrays

Move the keys into static arrays. This will avoid copying the keys
from the code section to the stack on the previous init_keys implementation.
Like this, the keys are hardcoded into data section at compile time,
and can be completely wiped after use.

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