mfd: vexpress: Add !ARCH_USES_GETTIMEOFFSET dependency

The MFD_VEXPRESS_SYSREG driver selects CLKSRC_MMIO, which in turn
conflicts with ARCH_USES_GETTIMEOFFSET, causing a harmless Kconfig
warning when it is set:

warning: (ARCH_MVEBU && ARCH_DIGICOLOR && ARCH_GEMINI && ARCH_KEYSTONE && ARCH_MOXART && ARCH_MXS && PLAT_SPEAR && ARCH_SUNXI && ARCH_TEGRA && ARCH_U300 && PLAT_ORION && ARCH_CLPS711X && ARCH_EP93XX && ARCH_NETX && ARCH_IXP4XX && ARCH_KS8695 && ARCH_W90X900 && ARCH_PXA && ARCH_SA1100 && ARCH_OMAP1 && ARCH_BCM_IPROC && ARCH_INTEGRATOR_AP && ARCH_OMAP2PLUS && MFD_VEXPRESS_SYSREG) selects CLKSRC_MMIO which has unmet direct dependencies (!ARCH_USES_GETTIMEOFFSET)

This was apparently hidden by the fact that no ARM platform that
still sets ARCH_USES_GETTIMEOFFSET has gpiolib support, and we
already have a dependency on GPIOLIB that I added a while ago.

However, after 296ad4ac ("gpio: remove deps on
ARCH_[WANT_OPTIONAL|REQUIRE]_GPIOLIB"), any platform can enable
CONFIG_GPIOLIB, and that lets us enable MFD_VEXPRESS_SYSREG
as well.

This adds an explicit dependency on !ARCH_USES_GETTIMEOFFSET
to guarantee that we can enable the CLKSRC_MMIO driver without
getting warnings.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>