hw/gpio: Fix property accessors of the AST2600 GPIO 1.8V model

hw/gpio: Fix property accessors of the AST2600 GPIO 1.8V model

The property names of AST2600 GPIO 1.8V model are one character bigger
than the names of the other ASPEED GPIO model. Increase the string
buffer size by one and be more strict on the expected pattern of the
property name.

This fixes the QOM test of the ast2600-evb machine under :

  Apple LLVM version 10.0.0 (clang-1000.10.44.4)
  Target: x86_64-apple-darwin17.7.0
  Thread model: posix
  InstalledDir: /Library/Developer/CommandLineTools/usr/bin

Cc: Rashmica Gupta <rashmica.g@gmail.com>
Fixes: 36d737ee82b2 ("hw/gpio: Add in AST2600 specific implementation")
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-id: 20191023130455.1347-2-clg@kaod.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>