MdePkg: Allow PcdFSBClock to by Dynamic

We are going to want to change the value of PcdFSBClock at run time in
OvmfXen, so move it to the PcdsDynamic section.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2490


Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20210412133003.146438-3-anthony.perard@citrix.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>