of: add missing major vendors

Looking at the current vendor strings used in the kernel's .dts/.dtsi
files, some vendors are used a high number of times without
being documented. Document the ones that are used more than 10 times.

Note: a few inconsistencies were found, and thus not documented.
Here is the list:
- mrvl: duplicates "marvell"
- st-ericsson: duplicates "ste" _and_ "stericsson"
- pci8086: seems to be a unfortunate alias for "intel"
- pnpPNP: used on PowerPC?

Signed-off-by: Florian Vaussard <florian.vaussard@epfl.ch>
Signed-off-by: Rob Herring <robh@kernel.org>