tracing/boot: Support multiple histograms for each event

Add multiple histograms support for each event. This allows
user to set multiple histograms to an event.

ftrace.[instance.INSTANCE.]event.GROUP.EVENT.hist[.N] {
...
}

The 'N' is a digit started string and it can be omitted
for the default histogram.

For example, multiple hist triggers example in the
Documentation/trace/histogram.rst can be written as below;

ftrace.event.net.netif_receive_skb.hist {
1 {
keys = skbaddr.hex
values = len
filter = len < 0
}
2 {
keys = skbaddr.hex
values = len
filter = len > 4096
}
3 {
keys = skbaddr.hex
values = len
filter = len == 256
}
4 {
keys = skbaddr.hex
values = len
}
5 {
keys = len
values = common_preempt_count
}
}

Link: https://lkml.kernel.org/r/162856125628.203126.15846930277378572120.stgit@devnote2

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>