dt-bindings: gpio: aspeed: Remove reference to clock name

HPLL is in fact not the clock we need. Remove prescription of which clock to
avoid further error. Please refer to your datasheet and double check like I
should have.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>