stm32mp1: select boot device and partition

Bootrom loads SPL from SDCARD or eMMC
according BootPin selection.

Then SPL loads U-Boot on the same mmc device
with the following predefined GPT partitioning:

on SDCARD: gpt partitioning
  1: SPL
  2: SPL#2
  3: U-Boot
  4: bootable partition

on eMMC:
  The 2 boot partitions are used for SPL (2 copy)
    boot1: SPL
    boot2: SPL#2
  The user partition use gpt partitioning
    1: U-Boot
    2: bootable partition

This patch select the correct SPL partition
(3 for SDCARD on mmc0 and 1 for eMMC on mmc1)
according the BootRom information saved in TAMP register
and based on configuration flasg:
- CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_PARTITION
  => for BOOT_DEVICE_MMC1 or mmc 0 in U-Boot
- CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_PARTITION_MMC2 (new)
  => for BOOT_DEVICE_MMC2 or mmc 1 in U-Boot

And the correct boot_targets is selected according the environment
variables boot_device and boot_instance, with preboot command,
to search the bootable partition with kernel on this device
(generic distro support).

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>