MIPS: Alchemy: platform: use clk framework for uarts

Use the clock framework to get the rate of the peripheral clock.
Remove the now obsolete get_uart_baud_base function.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Cc: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/7468/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>