powerpc/fsl: try to explain why the interrupt numbers are off by 16

Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>

Documentation/powerpc/dts-bindings/fsl/mpic.txt

+* OpenPIC and its interrupt numbers on Freescale's e500/e600 cores
+
+The OpenPIC specification does not specify which interrupt source has to
+become which interrupt number. This is up to the software implementation
+of the interrupt controller. The only requirement is that every
+interrupt source has to have an unique interrupt number / vector number.
+To accomplish this the current implementation assigns the number zero to
+the first source, the number one to the second source and so on until
+all interrupt sources have their unique number.
+Usually the assigned vector number equals the interrupt number mentioned
+in the documentation for a given core / CPU. This is however not true
+for the e500 cores (MPC85XX CPUs) where the documentation distinguishes
+between internal and external interrupt sources and starts counting at
+zero for both of them.
+
+So what to write for external interrupt source X or internal interrupt
+source Y into the device tree? Here is an example:
+
+The memory map for the interrupt controller in the MPC8544[0] shows,
+that the first interrupt source starts at 0x5_0000 (PIC Register Address
+Map-Interrupt Source Configuration Registers). This source becomes the
+number zero therefore:
+ External interrupt 0 = interrupt number 0
+ External interrupt 1 = interrupt number 1
+ External interrupt 2 = interrupt number 2
+ ...
+Every interrupt number allocates 0x20 bytes register space. So to get
+its number it is sufficient to shift the lower 16bits to right by five.
+So for the external interrupt 10 we have:
+  0x0140 >> 5 = 10
+
+After the external sources, the internal sources follow. The in core I2C
+controller on the MPC8544 for instance has the internal source number
+27. Oo obtain its interrupt number we take the lower 16bits of its memory
+address (0x5_0560) and shift it right:
+ 0x0560 >> 5 = 43
+
+Therefore the I2C device node for the MPC8544 CPU has to have the
+interrupt number 43 specified in the device tree.
+
+[0] MPC8544E PowerQUICCTM III, Integrated Host Processor Family Reference Manual
+    MPC8544ERM Rev. 1 10/2007