ShellPkg/Pci.c: Update supported link speed to PCI5.0

ShellPkg/Pci.c: Update supported link speed to PCI5.0

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1955

Refer to PCI express base specification Reversion 5.0, Version
1.0, Table 7-33, Supported Link Speeds Vector bit 3 indicate
the speed 16 GT/s and bit 4 indicate the speed 32 GT/s.
Add the support to shell command 'pci ...'.

Change the MaxLinkSpeed other values' result from 'Unknown'
to 'Reserved' to make the result align.

Cc: Ray Ni <ray.ni@intel.com>
Cc: Oleksiy <oleksiyy@ami.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Zhichao Gao <zhichao.gao@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>