pinctrl: aspeed: Describe the heartbeat function on ball Y23

pinctrl: aspeed: Describe the heartbeat function on ball Y23

The default pinmux configuration for Y23 is to route a heartbeat to
drive a LED. Previous revisions of the AST2600 datasheet did not include
a description of this function.

Fixes: 2eda1cdec49f ("pinctrl: aspeed: Add AST2600 pinmux support")
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20200701030756.2834657-1-joel@jms.id.au
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>