i.MX6: mx6qsabrelite: README: don't pass chip-select to sf probe command

i.MX6: mx6qsabrelite: README: don't pass chip-select to sf probe command

board/freescale/mx6qsabrelite/README explain a procedure to
update the SPI-NOR on the SabreLite board without Freescale
manufacturing tool but following this procedure leads to both
"sf erase" and "sf write" failing on a mx6qsabrelite board:

MX6QSABRELITE U-Boot > sf probe 1
MX6QSABRELITE U-Boot > sf erase 0 0x40000
SPI flash erase failed
MX6QSABRELITE U-Boot > sf write 0x10800000 0 0x40000
SPI flash write failed

This is because the chip-select 1 is wrong and the correct
value is 0x7300.

Since commit c1173bd0 ("sf command: allow default bus and chip selects")
the chip-select and bus arguments for the sf probe command are optional
so let's just remove it and use "sf probe" instead.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>