ShellPkg: acpiview: GTDT: Remove redundant forward declarations

ShellPkg: acpiview: GTDT: Remove redundant forward declarations

Remove redundant forward function declarations by repositioning
blocks of code. This way the code structure is consistent across
ACPI table parsers and the code becomes more concise.

Replace multple use of literal values for GT Block Timer Frame
count/number validation with a macro definition.

Signed-off-by: Krzysztof Koch <krzysztof.koch@arm.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Alexei Fedorov <Alexei.Fedorov@arm.com>
Reviewed-by: Zhichao Gao <zhichao.gao@intel.com>