cmake: Fix CMAKE_SYSTEM_PROCESSOR setting for SDK

cmake: Fix CMAKE_SYSTEM_PROCESSOR setting for SDK

When building using an SDK, cmake complains that the target
architecture 'cortexa53-crypto' is unknown. The same build in bitbake
uses the target architecture 'aarch64'.

Set CMAKE_SYSTEM_PROCESSOR the same as for bitbake.

Signed-off-by: Tom Hochstein <tom.hochstein@nxp.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>