ARM: stm32mp: add entry point, not point.pblb to pblb-y ...

Other images/Makefile.${arch}s, e.g. images/Makefile.imx, populate
pblp-* with the entry points to build, not the pbl file that's
generated. Adjust the images/Makefile.stm32mp  to do the same.

Cc: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

ARM: stm32mp1: rename to stm32mp ...

Serial and clk driver both depend on CONFIG_ARCH_STM32MP1,
so either the Kconfig symbol or their depend needs to change.

Patches posted by the vendor to Linux, U-Boot and their BSP
Yocto-Layer speak of a STM32MP-Family of which the STM32MP1
is the first series, thus rename the arch by dropping the 1.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>