It appears that all i.MX51 and i.MX6 based boards that support early
debug output inevitably do exactly the same thing with the registers
of the UART peripheral. So to avoid code duplication three new
subroutines are introduced:

  - imx_uart_setup_ll() that allows user to perform aforementioned
    "same thing" on any arbitrary UART at 'uartbase' and clocked at
    'refclock' Hz.

  - imx6_uart_setup_ll() and imx5_uart_setup_ll() that do those
    actions assuming UART to be the one specified in configuration and
    'refclock' to be equal to what it is when SoC comes out of reset.

NOTE: Please note that 'imx*_uart_setup_ll' functions do add a slight
difference in behaviour by dropping the initialization of ONEMS and
UESC registers since those do not seem to be needed for early UART
functionality

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>