ARM: dts: uniphier: rename support card serial node to fix schema warning

Since commit e69f5dc6 ("dt-bindings: serial: Convert 8250 to
json-schema"), the schema for "ns16550a" is checked.

'make ARCH=arm dtbs_check' emits the following warning:

  uart@b0000: $nodename:0: 'uart@b0000' does not match '^serial(@[0-9a-f,]+)*$'

Rename the node to follow the pattern defined in
Documentation/devicetree/bindings/serial/serial.yaml

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>