lmbench: fix lmbench memory check failure

lmbench can't proceed on some targets.  The memory check fails because the
memory latency of each page is longer then 10us, which is a time limit set
in the original memsize.c.

The memory latency is very different on different targets due to the
hardware and current system load.  The targets with slower memory
chips or heavy system load need much longer time to read or write
the memory.

This fix changes the fixed time limit of 10us to a specific value calculated
from the runtime target.

Also set an upper limit of memory size used for lmbench testing.  The memory
check sometimes fails if the target has a large amount of memory, for
example more than 4G.

Signed-off-by: Qingming Su <qingming.su@windriver.com>
Signed-off-by: Fupan Li <fupan.li@windriver.com>
Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>