ARM: OMAP2+: powerdomain: fix whitespace, improve flag comments

Fix some whitespace problems introduced by commit
da03ce65 ("OMAP3: powerdomain data:
add voltage domains").  Also, improve the documentation for the struct
powerdomain.flags field.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>