boards: arm: mps2-an521: Fix DT memory regions

Currently RAM region specified in the DT for board mps2-an512 to store
data (not to run code) is set to start at 0x3000_0000 and a 16M
contiguous space is assumed. However, at that address there is no such
contiguous space of 16M, rather only a 128K area is available. As a
consequence large applications linked with Zephyr might end up using
memory regions that are not valid, specially at runtime when the stack
grows, causing a BusFault.

Application Note 512 only specifies a 16M contiguous space available
starting at 0x8000_0000 (please see 'Table 3-4: SSRAM2 and SSRAM3
address mapping' and 'Table 3-6: External PSRAM mapping to Code Memory',
on pages 3-7 and 3-8, respectively), which resides in the PSRAM
(external RAM).

The AN521 also specifies a 4M contiguous space available starting at
0x3800_0000 which can be used as RAM for data storage and which is not
currently described in the DT.

The current DT also defines a 224M flash region (to run code) which
doesn't effectively exist, because most of it is reserved (~148M).

That commit fixes the incorrect definition of region 0x3000_0000 (16M)
and hence defines a new region called 'sram2_3' that maps to region
0x3800_0000 (4M) which is used as RAM to store data, and fixes the flash
region defining a new region 'sram1' (4M) from where code is executed
(starting at 0x1000_0000). The board has no real flash memory, rather an
auxilary HW populates the appropriate memory regions from images found
in a MicroSD card.

That commit also defines the missing PSRAM (16M) region ('psram') which
can be used by large programs as a general purpose RAM.

Finally, it also fixes the DT for the non-secure memory regions to
reflect the fixes described above for the secure memory regions.

Signed-off-by: Gustavo Romero <gustavo.romero@linaro.org>