gpio: lpc18xx: add struct device local variable

This is a non-functional change, it simplifies multiple access to
'struct device' pointer derived from a platform device pointer,
the new local variable will also be used in the following changes.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>