refactor(stm32mp1): use a macro for header size

Use STM32MP_HEADER_RESERVED_SIZE macro instead of a fixed value 0x3000
in linker script.

Change-Id: I2702285c15aebaa1304a891c8aaabc949a912ba6
Signed-off-by: Yann Gautier <yann.gautier@st.com>
diff --git a/plat/st/stm32mp1/stm32mp1.ld.S b/plat/st/stm32mp1/stm32mp1.ld.S
index 23716ac..2254fee 100644
--- a/plat/st/stm32mp1/stm32mp1.ld.S
+++ b/plat/st/stm32mp1/stm32mp1.ld.S
@@ -16,7 +16,7 @@
 ENTRY(__BL2_IMAGE_START__)
 
 MEMORY {
-	HEADER (rw) : ORIGIN = 0x00000000, LENGTH = 0x3000
+	HEADER (rw) : ORIGIN = 0x00000000, LENGTH = STM32MP_HEADER_RESERVED_SIZE
 	RAM (rwx) : ORIGIN = STM32MP_BINARY_BASE, LENGTH = STM32MP_BINARY_SIZE
 }