board/vpac270/onenand.c: Fix build errors

Building for vpac270_ond_256 configuration fails:

arch/arm/lib/libarm.o: In function `icache_disable':
/home/ag/git/u-boot/arch/arm/lib/cache-cp15.c:156: multiple
definition of `icache_disable'
board/vpac270/libvpac270.o:/home/ag/git/u-boot/board/vpac270/onenand.c:65:
first defined here
arch/arm/lib/libarm.o: In function `dcache_disable':
/home/ag/git/u-boot/arch/arm/lib/cache-cp15.c:188: multiple
definition of `dcache_disable'
board/vpac270/libvpac270.o:/home/ag/git/u-boot/board/vpac270/onenand.c:66:
first defined here
make[1]: *** [/home/ag/git/u-boot/spl/u-boot-spl] Error 1

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Acked-by: Marek Vasut <marex@denx.de>