arm: mx5: Fix memory slowness on MX53QSB

arm: mx5: Fix memory slowness on MX53QSB

Fix memory access slowness on i.MX53 MX53QSB board. Let us inspect the
issue: First of all, the i.MX53 CPU has two memory banks mapped at
0x7000_0000 and 0xb000_0000 and each of those can hold up to 1GiB of
DRAM memory. Notice that the memory area is not continuous. On MX53QSB,
each of the banks contain 512MiB of DRAM, which makes a total of 1GiB
of memory available to the system.

The problem is how the relocation of U-Boot is treated on i.MX53 . The
U-Boot is placed at the ((start of first DRAM partition) + (gd->ram_size)) .
This in turn poses a problem, since in our case, the gd->ram_size is 1GiB,
the first DRAM bank starts at 0x7000_0000 and contains 512MiB of memory.
Thus, with this algorithm, U-Boot is placed at offset:

    0x7000_0000 + 1GiB - sizeof(u-boot and some small margin)

This is past the DRAM available in the first bank on MX53QSB, but is still
within the address range of the first DRAM bank. Because of the memory
wrap-around, the data can still be read and written to this area, but the
access is much slower.

There were two ideas how to solve this problem, first was to map both of
the available DRAM chunks next to one another by using MMU, second was to
define CONFIG_VERY_BIG_RAM and CONFIG_MAX_MEM_MAPPED to size of the memory
in the first DRAM bank. We choose the later because it turns out the former
is not applicable afterall. The former cannot be used in case Linux kernel
was loaded into the second DRAM bank area, which would be remapped and one
would try booting the kernel, since at some point before the kernel is started,
the MMU would be turned off, which would destroy the mapping and hang the
system.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Wolfgang Denk <wd@denx.de>