[mlir][openacc] Add acc.kernels operation

The acc.kernels operation models the OpenACC kernels construct.
The kernels construct defines a region of a program that is
compiled into a sequence of kernels to be executed on the current device.
The operation is modelled on the acc.parallel operation and will
receive similar updates when the data operands operations will
be implemented.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D148277