riscv/ftrace: Add dynamic function graph tracer support

Once the function_graph tracer is enabled, a filtered function has the
following call sequence:

* ftracer_caller         ==> on/off by ftrace_make_call/ftrace_make_nop
* ftrace_graph_caller
* ftrace_graph_call      ==> on/off by ftrace_en/disable_ftrace_graph_caller
* prepare_ftrace_return

Considering the following DYNAMIC_FTRACE_WITH_REGS feature, it would be
more extendable to have a ftrace_graph_caller function, instead of
calling prepare_ftrace_return directly in ftrace_caller.

Cc: Greentime Hu <greentime@andestech.com>
Signed-off-by: Alan Kao <alankao@andestech.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>