rockchip: rk3399: update SPL_STACK_R_ADDR

Use the same SPL_STACK_R_ADDR in Kconfig instead of each board config;
default to 0x4000000(64MB) instead of 0x80000(512KB) for this address
can support all the SoCs including those may have only 64MB memory, and
also reserve enough space for atf, kernel(in falcon mode) loading.

After the ATF entry move to 0x40000, the stack from 0x80000 may be override
when loading ATF bl31.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>