Make Broadcom XLR use shared ds1374 RTC driver

[kbowling]

Summary

XLP shares the same driver, looks like it was added to iicbus in r233537

[sbruno]

Looks good from a compile perspective, all 3 kernels compile here.

make -s -j8 buildkernel KERNCONF=XLRN32 TARGET=mips TARGET_ARCH=mipsn32
make -s -j8 buildkernel KERNCONF=XLRN64 TARGET=mips TARGET_ARCH=mips64
make -s -j8 buildkernel KERNCONF=XLR TARGET=mips TARGET_ARCH=mips

[sbruno]

Committed at svn r286015