mach-stm32: Fix MPU region size dedicated to SDRAM for STM32F4

mach-stm32: Fix MPU region size dedicated to SDRAM for STM32F4

The MPU region dedicated for SDRAM for STM32F4 SoCs family
was set to 16MB, but STM32F429 Evaluation board have 32MB of SDRAM.

When kernel starts, only first 16MB of SDRAM are configured with XN
(eXecute Never) bit disabled, whereas kernel is using 32MB.
To avoid such situation in the future, extend this MPU region to 512MB
as for STM32F7/H7.

It fixes the following user land exception on STM32F429 Evaluation
board :

[    1.713002] VFS: Mounted root (ext4 filesystem) readonly on device 179:2.
[    1.722605] devtmpfs: mounted
[    1.733057] Freeing unused kernel memory: 72K
[    1.737622] This architecture does not have kernel memory protection.
[    1.744070] Run /sbin/init as init process
[    1.906850]
[    1.906850] Unhandled exception: IPSR = 00000004 LR = fffffffd
[    1.914282] CPU: 0 PID: 1 Comm: init Not tainted 5.1.0-00002-gcf9ca5719954 #6
[    1.921433] Hardware name: STM32 (Device Tree Support)
[    1.926601] PC is at 0x1a00b64
[    1.929642] LR is at   (null)
[    1.932669] pc : [<01a00b64>]    lr : [<00000000>]    psr: 01000000
[    1.938993] sp : 01a5cfb0  ip : 00000000  fp : 00000000
[    1.944269] r10: 01a43b00  r9 : 00000000  r8 : 00000000
[    1.949564] r7 : 00000000  r6 : 00000000  r5 : 00000000  r4 : 00000000
[    1.956168] r3 : 00000000  r2 : 00000000  r1 : 00000000  r0 : 00000000
[    1.962701] xPSR: 01000000
[    1.965506] CPU: 0 PID: 1 Comm: init Not tainted 5.1.0-00002-gcf9ca5719954 #6
[    1.972658] Hardware name: STM32 (Device Tree Support)
[    1.978132] [<0000c009>] (unwind_backtrace) from [<0000b24f>] (show_stack+0xb/0xc)
[    1.986024] [<0000b24f>] (show_stack) from [<0000b947>] (__invalid_entry+0x4b/0x4c)

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>