ARM: bcm283x: Add BCM283x_BASE define

Devices of bcm283x have different base address, depending if they are on
bcm2835 or bcm2836/7. Use BCM283x_BASE depending on the SoC you want to
build and only add the offset in the header files.

Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Signed-off-by: Andrei Gherzan <andrei@balena.io>