pinctrl: aspeed: Add implementation-related documentation

The ASPEED pinctrl driver implementations make heavy use of macros to
minimise tedium of implementation and maximise the chance that the
compiler will catch errors in defining signal and pin configurations.
While the goal of minimising errors is achieved, it is at the cost of
the complexity of the macros.

Document examples of the expanded form of pin declarations to
demonstrate the operation of the macros.

Cc: Johnny Huang <johnny_huang@aspeedtech.com>
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20190628023838.15426-9-andrew@aj.id.au
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

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