mfd: vexpress-sysreg: Drop selecting CONFIG_CLKSRC_MMIO

Nothing in the VExpress sysregs nor the MFD child drivers use
CONFIG_CLKSRC_MMIO. There's the 24MHz counter, but that's handled by
drivers/clocksource/timer-versatile.c which doesn't use
CONFIG_CLKSRC_MMIO either. So let's just drop CONFIG_CLKSRC_MMIO.

As the !ARCH_USES_GETTIMEOFFSET dependency was added for
CONFIG_CLKSRC_MMIO, that can be dropped, too.

Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>