Use OF_getencprop over OF_getprop and fdt32_to_cpu. The latter may give
us the wrong data in the failure case if shift was not zero.

Remove the extra copy of uart_fdt_get_clock and uart_fdt_get_shift. While
here also use OF_getencprop in uart_fdt_get_clock.

Move uart_fdt_get_clock and uart_fdt_get_shift to uart_bus_fdt.c, we may
not build uart_cpu_fdt.c in all configs.

Fix uart_fdt_get_clock. It should have been using the cell variable passed
in, not value on the stack.