dm: core: describe uclass_root_s

'make htmldocs' creates a warning:

./include/asm-generic/global_data.h:443:
warning: Function parameter or member 'uclass_root_s'
not described in 'global_data'

Correct the member descriptions.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>