Use a dedicated UART config(CONFIG_SHAKTI_UART) to select
shakti uart.

Signed-off-by: Vijai Kumar K <vijai@behindbytes.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210731190229.137483-1-vijai@behindbytes.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>