serial: Rework CONFIG_SYS_BAUDRATE_TABLE

In order to move CONFIG_SYS_BAUDRATE_TABLE to Kconfig, we need to rework
the logic a bit.  Rename the users of CONFIG_SYS_BAUDRATE_TABLE to
SYS_BAUDRATE_TABLE.  Introduce a series of CONFIG_BAUDRATE_TABLE_...
that include some number of baud rates.  These match all existing users.
The help for each entry specifies what the exact table is, for a given
option.  Define what SYS_BAUDRATE_TABLE will be in include/serial.h now.

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