sf command: allow default bus and chip selects

This patch allows a board configuration file to provide default bus
and chip-selects for SPI flash so that first argument to the 'sf' command
is optional.

On boards that use the mxc_spi driver and a GPIO for chip select, this allows
a much simpler command line:
	U-Boot> sf probe
instead of
	U-Boot> sf probe 0x5300

Tested-by: Jason Liu <jason.hui@linaro.org>
Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>