serial: a37xx: Do not call get_ref_clk() in _debug_uart_init()

Static inline function _debug_uart_init() should avoid calling external
(non-inline) functions. Therefore do not call get_ref_clk() in
_debug_uart_init() and reimplement its functionality without external
function calls.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>