gpio: brcmstb: Use dynamic GPIO base numbers

Forcing a gpiochip to have a fixed base number now leads to a warning
message. Remove the need to do so by calculating hwirq numbers based
on bank numbers.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>
Fixes: 3b0213d5 ("gpio: Add GPIO support for Broadcom STB SoCs")