hikey: configure 4 MB of secure DRAM for OP-TEE Secure Data Path

hikey: configure 4 MB of secure DRAM for OP-TEE Secure Data Path

Update the memory firewall configuration to reserve 4 MB of secure RAM
for use by the kernel and OP-TEE as the Secure Data Path pool.
Note that this address range (0x3E800000 - 0x3EC00000) falls in the
range already set aside by UEFI (which reserves the upper 32 MB of the
1GB DRAM for OP-TEE [1]) and was previously unused.

[1] https://github.com/96boards-hikey/edk2/blob/hikey/HisiPkg/HiKeyPkg/Library/HiKeyLib/HiKeyMem.c#L44
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Acked-by: Victor Chong <victor.chong@linaro.org>