ARM: 9457/1: ftrace: Implement HAVE_FUNCTION_GRAPH_FREGS

Enable support for ftrace's funcgraph-retval feature by capturing r0-r3
and fp.
Since ARM does not provide its own __arch_ftrace_regs structure,
we instead populate pt_regs with the registers required by ftrace.

Cc: Donglin Peng <pengdonglin@sangfor.com.cn>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>