BCM4908 has controller that needs different IRQ handling just like the
BCM63138. Describe it properly.

On Linux this change fixes:
brcmstb_nand ff801800.nand: timeout waiting for command 0x9
brcmstb_nand ff801800.nand: intfc status d0000000
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>