ARM: dts: stm32mp13: Add support of ck_usbo_48m in pre-reloc stage

ARM: dts: stm32mp13: Add support of ck_usbo_48m in pre-reloc stage

The clock ck_usbo_48m is a clock source for RCC, so the ck_usbo_48m
clock provided by usbphyc need to be probed when RCC clock driver is
required, in pre-reloc stage.

This patch allow to remove the following warning:

clk_register: failed to get ck_usbo_48m device (parent of usbo_k)

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>