MPC8315ERD: fix build error

Commit 29c6fbe0 "MPC5121: Add USB EHCI support" renamed
CONFIG_SYS_MPC8xxx_USB_ADDR into CONFIG_SYS_FSL_USB_ADDR but missed
to update arch/powerpc/cpu/mpc83xx/cpu_init.c, resulting in:

cpu_init.c: In function 'cpu_init_f':
cpu_init.c:332: error: 'CONFIG_SYS_MPC8xxx_USB_ADDR' undeclared (first use in this function)
cpu_init.c:332: error: (Each undeclared identifier is reported only once
cpu_init.c:332: error: for each function it appears in.)
make[1]: *** [/work/wd/tmp-ppc/arch/powerpc/cpu/mpc83xx/cpu_init.o] Error 1

Fix this.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Kim Phillips <kim.phillips@freescale.com>