[platform][zynq] fix uart driver
[platform][zynq] initialize more of the uart

no longer relies on the target specific jam table except for
baud rate calculations.
[platform][zynq] update clock setting api, print boot reason
[arch][arm] add documentation for exceptions.S and start.S
README.md: exdend readme file

Add information about how to build LK, customizing the build using
local.mk, and how build system works.
README.md: add 'how to contribute' section. Rename realview-pb

Realview-PB has been removed from the tree. Use vexpress-a9 project as an
example in "building" section.

Give suggestions for contributors in 'how to contribute' section. These are not
strict rules but rather practices to make code merging easier.
[arch][arm] exceptions.S: remove documentation for ARM instructions

Inline documentation of ARM instructions on how the stack is created decreases
the readability of the code. It is better to include this in the complementary
document.
[doc] add internals.md

This document is an attempt to describe the internals of LK for ARM
architecture. It is aimed at people who are new to LK source. In some parts,
detailed explanation is given regarding to the architecture and instructions so
that people who are new to ARM assembly are not get lost.