x86: qemu: enable CONFIG_SPL_DM_RTC

Since commit 380d4f78 ("rtc: Allow use of RTC in SPL and TPL")
qemu-x86_64_defconfig does not boot anymore.

Fixes: 380d4f78 ("rtc: Allow use of RTC in SPL and TPL")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>