pinctrl: samsung: Document required order of banks

This patch documents requirement coming from the way
exynos_eint_gpio_irq() is working now, which expects EINTG banks to be
at the beginning of the bank arrays.

Signed-off-by: Paweł Chmiel <pawel.mikolaj.chmiel@gmail.com>
Reviewed-by: Tomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>