arm64: dts: exynos: Fix silent hang after boot on Espresso

Once regulators are disabled after kernel boot, on Espresso board silent
hang observed because of LDO7 being disabled.  LDO7 actually provide
power to CPU cores and non-cpu blocks circuitries.  Keep this regulator
always-on to fix this hang.

Fixes: 9589f772 ("arm64: dts: Add S2MPS15 PMIC node on exynos7-espresso")
Signed-off-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>