EmbeddedPkg/AcpiLib: fix SBSA Generic Watchdog helper definition

EmbeddedPkg/AcpiLib: fix SBSA Generic Watchdog helper definition

The Reserved field in the SBSA Generic Watchdog Structure is 1 byte in
length. Refer Table 5-123 in the ACPI 5.1 Specification Errata A.

The EFI_ACPI_5_1_SBSA_GENERIC_WATCHDOG_STRUCTURE_INIT() helper
macro was initializing this field as EFI_ACPI_RESERVED_WORD instead of
EFI_ACPI_RESERVED_BYTE.

Although this does not cause any functional issue; it does not comply
with the specification.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Evan Lloyd <evan.lloyd@arm.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>