lpc32xx: Kconfig: switch to CONFIG_CONS_INDEX

There's nothing special or unique to the lpc32xx that requires its own config
parameter for specifying the console uart index. Therefore instead of using
the lpc32xx-specific CONFIG_SYS_LPC32XX_UART include parameter, use the
already-available CONFIG_CONS_INDEX from Kconfig.

Signed-off-by: Trevor Woerner <twoerner@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>