lpuart: add a get_lpuart_clk function

It's not always true that LPUART clock is CONFIG_SYS_CLK_FREQ. This
patch provides a weak function get_lpuart_clk(), so that the clock
can be ovreridden on a specific board which uses different clock
for LPUART.

Signed-off-by: Shaohui Xie <Shaohui.Xie@nxp.com>
[YS: Reformat commit message]
Reviewed-by: York Sun <york.sun@nxp.com>