rpi3: enlarge SEC_DRAM0_SIZE for optee_test to pass

Running optee_test failed because SEC_DRAM0_SIZE is too small. Previous
is 2 MB. We enlarge it to 11 MB for passing the test. Also we reduce
the NS_DRAM0_SIZE from 13MB to 4MB so that the whole section is still
fit in 16MB.

This commit also modified the document to reflect the changes we've
made in code.

Tested-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
Signed-off-by: Ying-Chun Liu (PaulLiu) <paulliu@debian.org>