Keystone2 Lamar SoC uses the same keystone SerDes driver.
All Keystone2 EVM boards currently use SerDes driver, so move
CONFIG_TI_KEYSTONE_SERDES to common configuration file.
Acked-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>