ARM: dts: exynos: Use bustop PLL as the source for MMC clocks on Exynos5260

By default the MMC clock will be derived from mediatop PLL, which
usually runs at 666MHz. However as most SD and MMC clocks are multiples
or fractions of 100MHz, it makes more sense to use the bustop PLL
which runs at 800MHz. This matches the behaviour of the Samsung vendor
supplied 3.4 kernel.

Signed-off-by: Stuart Menefy <stuart.menefy@mathembedded.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>