irqchip/irq-brcmstb-l2: Add config for 2711 controller

We currently see these regularly:
[   25.157560] irq 31, desc: 00000000c15e6d2c, depth: 0, count: 0, unhandled: 0
[   25.164658] ->handle_irq():  00000000b1775675, brcmstb_l2_intc_irq_handle+0x0/0x1a8
[   25.172352] ->irq_data.chip(): 00000000fea59f1c, gic_chip_mode1+0x0/0x108
[   25.179166] ->action(): 000000003eda6d6f
[   25.183096] ->action->handler(): 000000002c09e646, bad_chained_irq+0x0/0x58
[   25.190084]      IRQ_LEVEL set
[   25.193142]    IRQ_NOPROBE set
[   25.196198]  IRQ_NOREQUEST set
[   25.199255]   IRQ_NOTHREAD set

with:
$ cat /proc/interrupts  | grep 31:
 31:          1          0          0          0     GICv2 129 Level     (null)

The interrupt is described in DT with IRQ_TYPE_LEVEL_HIGH

But the current compatible string uses the controller in edge triggered mode
(as that config matches our register layout).

Add a new compatible structure for level driven interrupt with our register layout.

We had already been using this compatible string in device tree, so no change needed
there.

Signed-off-by: Dom Cobley <popcornmix@gmail.com>