soc/fsl/qbman: Cleanup buffer pools if BMan was initialized prior to bootup

Clean the BMan buffer pools if the device had been initialized
previously.  This will ensure a consistent state if the kernel
was soft restarted (kexec for example)

Signed-off-by: Roy Pledge <roy.pledge@nxp.com>
Signed-off-by: Li Yang <leoyang.li@nxp.com>