ks2_evm: readme: add k2l evm board information

Currently Keystone2 Lamar evm (K2L) board is added, so update
Keystone2 readme file to have such one.

Acked-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>