README: Replace toolchain URL with reference to toolchain.mk
README: Replace toolchain URL with reference to toolchain.mk

This is to avoid hard coding toolchain URL which tends to change and
require constant updates.

Signed-off-by: Victor Chong <4aabb03bd6dd29c44a49684d87b08ac07e10b492@linaro.org>
Reviewed-by: Joakim Bech <741f90e2a7f4d9afbad8bd50be20560f4cf38962@linaro.org>

prime-zeng/optee_client