Add support for TBBR using ECDSA keys in ARM platforms

    - fixed compile error when KEY_ALG=ecdsa
    - add new option ecdsa for TF_MBEDTLS_KEY_ALG
    - add new option devel_ecdsa for ARM_ROTPK_LOCATION
    - add ecdsa key at plat/arm/board/common/rotpk/
    - reduce the mbedtls heap memory size to 13k

Change-Id: I3f7a6170af93fdbaaa7bf2fffb4680a9f6113c13
Signed-off-by: Qixiang Xu <qixiang.xu@arm.com>