powerpc/secvar: Use u64 in secvar_operations

There's no reason for secvar_operations to use uint64_t vs the more
common kernel type u64.

The types are compatible, but they require different printk format
strings which can lead to confusion.

Change all the secvar related routines to use u64.

Reviewed-by: Russell Currey <ruscur@russell.cc>
Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230210080401.345462-5-ajd@linux.ibm.com