Introduce lightweight BL platform parameter library ...

This patch adds some common helper code to support a lightweight
platform parameter passing framework between BLs that has already been
used on Rockchip platforms but is more widely useful to others as well.
It can be used as an implementation for the SoC firmware configuration
file mentioned in the docs, and is primarily intended for platforms
that only require a handful of values to be passed and want to get by
without a libfdt dependency. Parameters are stored in a linked list and
the parameter space is split in generic and vendor-specific parameter
types. Generic types will be handled by this code whereas
vendor-specific types have to be handled by a vendor-specific handler
function that gets passed in.

Change-Id: If3413d44e86b99d417294ce8d33eb2fc77a6183f
Signed-off-by: Julius Werner <jwerner@chromium.org>