stm32mp1: add macros to define PLAT_PARTITION_MAX_ENTRIES

stm32mp1: add macros to define PLAT_PARTITION_MAX_ENTRIES

There were fixed values when computing PLAT_PARTITION_MAX_ENTRIES.
Use STM32_BL33_PARTS_NUM and STM32_RUNTIME_PARTS_NUM. The first one is
for the number of copies of BL33. The second one depends on the use case
SP_min or OP-TEE. For OP-TEE, there are 3 partitions. For SP_min, as it
is in the same binary as BL2, it is set to 0. It will be set to 1 if
BL32 is in a separate binary.

Change-Id: Iba4d8ec5fbc713bebfbdcd9f9426c3fded20d3ad
Signed-off-by: Yann Gautier <yann.gautier@st.com>