MPC8xxx: Define cache ops for USB

MPC8xxx: Define cache ops for USB

This patch conditionally defines flush_dcache_range() and
invalidate_dcache_range() on MPC8xxx, to avoid EHCI complaining,
resulting in the following output:

$ ARCH=powerpc CROSS_COMPILE=powerpc-linux-gnu- ./MAKEALL MPC8572DS
Configuring for MPC8572DS board...
make: *** [u-boot] Error 1
powerpc-linux-gnu-size: './u-boot': No such file
e1000.c: In function ‘e1000_initialize’:
e1000.c:5264:13: warning: assignment from incompatible pointer type [enabled by default]
tsec.c: In function ‘tsec_initialize’:
tsec.c:638:12: warning: assignment from incompatible pointer type [enabled by default]
drivers/usb/host/libusb_host.o: In function `ehci_td_buffer':
/home/marex/U-Boot/u-boot-imx/drivers/usb/host/ehci-hcd.c:186: undefined reference to `flush_dcache_range'
drivers/usb/host/libusb_host.o: In function `ehci_submit_async':
/home/marex/U-Boot/u-boot-imx/drivers/usb/host/ehci-hcd.c:346: undefined reference to `flush_dcache_range'
/home/marex/U-Boot/u-boot-imx/drivers/usb/host/ehci-hcd.c:348: undefined reference to `flush_dcache_range'
/home/marex/U-Boot/u-boot-imx/drivers/usb/host/ehci-hcd.c:349: undefined reference to `flush_dcache_range'
/home/marex/U-Boot/u-boot-imx/drivers/usb/host/ehci-hcd.c:372: undefined reference to `invalidate_dcache_range'
/home/marex/U-Boot/u-boot-imx/drivers/usb/host/ehci-hcd.c:374: undefined reference to `invalidate_dcache_range'
/home/marex/U-Boot/u-boot-imx/drivers/usb/host/ehci-hcd.c:376: undefined reference to `invalidate_dcache_range'
/home/marex/U-Boot/u-boot-imx/drivers/usb/host/ehci-hcd.c:386: undefined reference to `invalidate_dcache_range'
make: *** [u-boot] Error 1

--------------------- SUMMARY ----------------------------
Boards compiled: 1
Boards with errors: 1 ( MPC8572DS )
----------------------------------------------------------

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Anatolij Gustschin <agust@denx.de>