powerpc: Use common of_kexec_alloc_and_setup_fdt()

powerpc: Use common of_kexec_alloc_and_setup_fdt()

The code for setting up the /chosen node in the device tree
and updating the memory reservation for the next kernel has been
moved to of_kexec_alloc_and_setup_fdt() defined in "drivers/of/kexec.c".

Use the common of_kexec_alloc_and_setup_fdt() to setup the device tree
and update the memory reservation for kexec for powerpc.

Signed-off-by: Rob Herring <[email protected]>
Signed-off-by: Lakshmi Ramasubramanian <[email protected]>
Reviewed-by: Thiago Jung Bauermann <[email protected]>
Signed-off-by: Rob Herring <[email protected]>
Link: https://lore.kernel.org/r/[email protected]