NetworkPkg: Fix bios bootup hang issue when enable network.

NetworkPkg: Fix bios bootup hang issue when enable network.

Dhcp6Dxe driver uses a new added dynamic PCD PcdDhcp6UidType in
NetworkPkg, but typically a pure UEFI driver are configured to
not allowed to access the dynamic PCDs in most platform, thus will
cause Dhcp6Dxe assert when it started. This patch add more PCD
access method types for module to use the PcdDhcp6UidType value
to fix the issue.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Zhang Lubo <lubo.zhang@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17986 6f19259b-4bc3-4df7-8a09-765794883524