tegra: nyan-big: Update CONFIG_SYS_TEXT to the default in README.chromium

tegra: nyan-big: Update CONFIG_SYS_TEXT to the default in README.chromium

To build U-Boot on a Nyan Big Chromebook the docs outline adjusting the Tegra124
defined CONFIG_SYS_TEXT_BASE but this has since been moved to individual config
files. We should have the default required for U-Boot chain loading on the
chromebook as the default CONFIG_SYS_TEXT_BASE and update the docs to remove
this now non required step.

Signed-off-by: Peter Robinson <[email protected]>
Reviewed-by: Simon Glass <[email protected]>
Signed-off-by: Tom Warren <[email protected]>