ARM: OMAP5+: GPIO: Add GPIO_TO_PIN() macro

GPIO_TO_PIN(bank, bank_gpio) returns the GPIO index
from the GPIO bank number and bank's GPIO offset number.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>