clk: mpc512x: remove migration support workarounds

this change removes workarounds which have become obsolete after
migration to common clock support has completed
- remove clkdev registration calls (compatibility clock item aliases)
  after all peripheral drivers were adjusted for device tree based
  clock lookup
- remove pre-enable workarounds after all peripheral drivers were
  adjusted to acquire their respective clock items

workarounds for these clock items get removed:  FEC (ethernet), I2C,
PSC (UART, SPI), PSC FIFO, USB, NFC (NAND flash), VIU (video capture),
BDLC (CAN), CAN MCLK, DIU (video output)

these clkdev registered names won't be provided any longer by the
MPC512x platform's clock driver:  "psc%d_mclk", "mscan%d_mclk",
"usb%d_clk", "nfc_clk", "viu_clk", "sys_clk", "ref_clk"

the pre-enable workaround for PCI remains, but depends on the presence
of PCI related device tree nodes (disables the PCI clock in the absence
of PCI nodes, keeps the PCI clock enabled in the presence of nodes) --
moving clock acquisition into the peripheral driver isn't possible for
PCI because its initialization takes place before the platform clock
driver gets initialized, thus the clock provider isn't available then

Cc: Mike Turquette <mturquette@linaro.org>
Cc: Anatolij Gustschin <agust@denx.de>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>