CONFIG_SYS_BAUDRATE_TABLE: Add <config_fallbacks.h>, place there

We provide a default table of { 9600, 19200, 38400, 57600, 115200 }
in <config_fallbacks.h> which mkconfig places after <configs/...h> in
the generated config file.  This is used when a board has not set its
own table.

Signed-off-by: Tom Rini <trini@ti.com>