Introduce gpio_export_array function to ease the export operation
for gpio arrays.
This can cooperate with the exist gpio_request_array function.

Signed-off-by: Dong Aisheng <dong.aisheng@linaro.org>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Arnd Bergmann <arnd@arndb.de>

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We already have gpio_request_array, i'm wondering if we can have
a gpio_export_array to cooperate to ease the export operation
for gpio arrays.
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