gpio: For ARC EM Starterkit, a 4-port GPIO implementation is being added

The file gpio_dw_registers.h already had the 4-port GPIO
registers defined, yet the gpio_dw.c implementation didn't
support it properly. There are 4 ports here, not 2, and only
PORTA can support interrupts and debounce.

On the em_starterkit board, for example, PORTA
has 3 bits for buttons: A, L and R. The other 3 ports should not
be used with interrupts & debounce.

I've re-worked this file to derive the port number from the
base address given. The lower 6 bits are divided by 12 to
derive the port number. From this, the registers EXT_PORTA,
EXT_PORTB, EXT_PORTC or EXT_PORTD can be read.

Also, for those ports that don't support interrupts,
set irq_num to 0, and that code will be avoided. I've verified
that I can access GPIO now correctly on the EM Starterkit. The
em_starterkit board support will be submitted soon but I'm
staging in this change first.

Change-Id: I98dbe083e03e046b40e07b4b14a99a39a6d0f0be
Signed-off-by: Chuck Jordan <cjordan@synopsys.com>