x86: broadwell: Move init of debug UART to cpu.c

x86: broadwell: Move init of debug UART to cpu.c

At present the debug UART is set up in sdram.c which is not the best place
since it has nothing in particular to do with SDRAM. Since we want to
support initing this in SPL too, move it to a common file.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: added 'broadwell' tag in the commit title]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>