malta: define CONFIG_MEMSIZE_IN_BYTES

The memsize environment variable must contain the
memory size in bytes on the Malta board. Otherwise
Linux will use wrong memory size which causes a kernel
panic.

Define CONFIG_MEMSIZE_IN_BYTES in malta.h to avoid
that.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>
Cc: Paul Burton <paul.burton@imgtec.com>