selftests/powerpc: Move bind_to_cpu() to utils.h

This function will be useful in the DSCR test patches later in this
series, so promote it to be shared by all powerpc selftests.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230406043320.125138-3-bgray@linux.ibm.com