irqchip: irq-bcm2836: Remove regmap and syscon use

The syscon node defines a register range that duplicates that used by
the local_intc node on bcm2836/7. Since irq-bcm2835 and irq-bcm2836 are
built in and always present together (both drivers are enabled by
CONFIG_ARCH_BCM2835), it is possible to replace the syscon usage with a
global variable that simplifies the code. Doing so does lose the
locking provided by regmap, but as only one side is using the regmap
interface (irq-bcm2835 uses readl and write) there is no loss of
atomicity.

See: https://github.com/raspberrypi/firmware/issues/926



Signed-off-by: Phil Elwell <phil@raspberrypi.org>