dts: bindings: serial: altera uart: remove redundent properties

A recent change added the stop-bits and data-bits to the base
uart-controller binding, meaning it's no longer required to
be added in the Altera specific binding.

This requires no further changes because its only use is in
uart_altera.c where only the index of the enum is used,
which remains the same between the new implementation and
how it was previously implemented in the altera specific binding.

Relevant commit: 0234f129

Signed-off-by: Hein Wessels <heinwessels93@gmail.com>