arm: highbank: Remove artificial SDRAM size

So far we were defining a somewhat confusing PHYS_SDRAM_1_SIZE variable,
which originally was only used for setting the memtest boundaries. This
definition in highbank.h has been removed about a year ago (moved to
Kconfig), so we also don't need the hard-coded size definition any longer.

Get rid of the misleading memory size definition, which was actually wrong
anyway (it's 4088 MB for those machines with just 4GB of DRAM).

Signed-off-by: Andre Przywara <andre.przywara@arm.com>