brcm47xx: switch to the kernel 4.14

brcm47xx: switch to the kernel 4.14

The biggest (and the only?) disadventage of this is obviously an
increased image size.

For mips74k the size of vmlinux goes up from 4186684 B to 4701436 B.
Most devices use LZMA compressed kernel so probably more important is
vmlinux.lzma size which goes up from 1342945 B to the 1508498 B.

Still this isn't something that should stop target kernel bump. There
are various adventages of kernel 4.14. If kernel / image size is a
serious concern for anyone, it's perfectly possible to use previous
release which is pretty solid for the brcm47xx.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Cc: Paul Wassi <p.wassi@gmx.at>
Cc: Hauke Mehrtens <hauke@hauke-m.de>