crypto/nx: Rename nx842_powernv_function as icswx function

Rename nx842_powernv_function to nx842_powernv_exec.
nx842_powernv_exec points to nx842_exec_icswx and
will be point to VAS exec function which will be added later
for P9 NX support.

Signed-off-by: Haren Myneni <haren@us.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>