ARM: ux500: Fix unmet direct dependency

A recent change to a Kconfig configuration saw MACH_U8500
remove TPS6105X selection. In doing so Kconfig stopped
selecting REGULATORS, which is still required. This patch
sees UX500_SOC_DB8500 explicitly select it instead.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>