rockchip: gpio: rk_gpio: use ROCKCHIP_GPIOS_PER_BANK as divider

rockchip: gpio: rk_gpio: use ROCKCHIP_GPIOS_PER_BANK as divider

The current divider to calculate the bank ID can change.
Given the Rockchip TRM not all gpio-banks have 32 pins per bank.
The "gpio-ranges" syntax allows multiple items with variable number
of pins. Use a constant ROCKCHIP_GPIOS_PER_BANK as fixed divider.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>