arm: dts: stm32mp1: use ssbl partition name for U-Boot

Continue to use the "ssbl" name for GPT partition of secondary boot
stage = U-Boot for basic boot with SPL to avoid to disturb existing user.

The "fip" partition name is only used for TFA_BOOT with FIP, it is a TF-A
BL2 requirement; it the default configuration for STMicroelectronics
boards.

Fixes: b73e8bf4 ("arm: stm32mp: add defconfig for trusted boot with FIP")
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>