ARM: nxp_imx: rt10xx: add XTAL and RTC_XTAL DT clocks

There are two external clocks driving i.MX RT 10xx SoCs: XTAL and
RTC_XTAL. Add "fixed-clock" DT nodes for each. Update also "nxp,imx-ccm"
binding, so that CCM (Clock Control Module) peripheral consumes those
clocks by "xtal" and "rtc-xtal" names.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>