ARM: OMAP2+: AM33XX: Add HWMOD_OMAP4_ZERO_CLKCTRL_OFFSET flag to rtc hwmod

The RTC hwmod on AM335x family of SoCs is unique in that the
clkctrl_offs in the PRCM is 0. We rely on a clkctrl_offs of zero as
indicating no clkctrl is present so we must flag this hwmod with
HWMOD_OMAP4_ZERO_CLKCTRL_OFFSET to indicate the 0 clkctrl_offs is in
fact valid in this case.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Acked-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>