serial: mpc52xx_uart: re-enable mpc5121 PSC UART support

Currently the support for MPC5121 PSC UART in the mpc52xx_uart
driver is broken (only console pre-initialized by the bootloader
works). Re-enable it now by providing MPC5121 specific ops
for PSCx clock activation, FIFO controller init/uninit and
MPC5121 PSC FIFO shared interrupt handling functions.

Signed-off-by: John Rigby <jcrigby@gmail.com>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>