powerpc/ftrace: Prepare PPC32's ftrace_caller() for CONFIG_DYNAMIC_FTRACE_WITH_ARGS

In order to implement CONFIG_DYNAMIC_FTRACE_WITH_ARGS, change
ftrace_caller() stack layout to match struct pt_regs.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/da9734eba504998fb914aca12131c9f6bf6120a8.1640017960.git.christophe.leroy@csgroup.eu