ARM: davinci: only select WT cache if cache is enabled

The DA830 chip only works if the dcache is in writethrough mode,
but that produces a harmless Kconfig warning if the cache happens
to be disabled:

warning: (ARCH_DAVINCI_DA830) selects CPU_DCACHE_WRITETHROUGH which has unmet direct dependencies ((CPU_ARM740T || CPU_ARM920T || CPU_ARM922T || CPU_ARM925T || CPU_ARM926T || CPU_ARM940T || CPU_ARM946E || CPU_ARM1020 || CPU_FA526) && !CPU_DCACHE_DISABLE)

This makes the select conditional so we don't have to worry
about the warning in randconfig builds any more.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>