hw/arm/omap1: Avoid unintended sign extension writing omap_rtc YEARS_REG

When writing to the YEARS_REG register, if the year value is
99 then the multiplication by 31536000 will overflow into
the sign bit of a 32 bit value and then be erroneously
sign-extended if time_t is 64 bits. Add a cast to avoid this.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
