reset: uniphier: fix USB clock line for LD20

For LD20, the bit 5 of the offset 0x200c turned out to be a USB3
reset.  The hardware document says it is the GIO reset despite LD20
has no GIO bus, confusingly.

Also, fix confusing comments for PXs3.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>