selftests/powerpc: Add automatically allocating read_file

A couple of tests roll their own auto-allocating file read logic.

Add a generic implementation and convert them to use it.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230203003947.38033-6-bgray@linux.ibm.com

selftests/powerpc: Add a rtas_filter selftest

Add a selftest to test the basic functionality of CONFIG_RTAS_FILTER.

Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
[mpe: Change rmo_start/end to 32-bit to avoid build errors on ppc64]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200820044512.7543-2-ajd@linux.ibm.com