ks2_evm: readme: align according to actual sources

Update readme file for Keystone II EVM boards to actual sources.
Also correct some typos. For now the Edison evaluation board is
added, README for K2E is mostly the same, so update README to
contain information also for K2E evm. Rename file to README as
it contains information for all keystone evm boards.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>