arm: mx5: Avoid hardcoding memory sizes on MX53QSB

The DRAM size can be easily detected at runtime on i.MX53. Implement
such detection on MX53QSB and adjust the rest of the macros accordingly
to use the detected values.

An important thing to note here is that we had to override the function
for trimming the effective DRAM address, get_effective_memsize(). That
is because the function uses CONFIG_MAX_MEM_MAPPED as the upper bound of
the available DRAM and we don't have gd->bd->bi_dram[0].size set up at
the time the function is called, thus we cannot put this into the macro
CONFIG_MAX_MEM_MAPPED . Instead, we use custom override where we use the
size of the first DRAM block which we just detected.

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>