MIPS: MSP71xx: Remove dead CONFIG_MTD_PMC_MSP_RAMROOT

CONFIG_MTD_PMC_MSP_RAMROOT doesn't exist in Kconfig, therefore removing all
references for it from the source code.

Signed-off-by: Christoph Egger <siccegge@cs.fau.de>
To:     Ralf Baechle <ralf@linux-mips.org>,
        Yoichi Yuasa <yuasa@linux-mips.org>, linux-mips@linux-mips.org,
        linux-kernel@vger.kernel.org
Cc:     vamos@i4.informatik.uni-erlangen.de
Acked-by: Shane McDonald <mcdonald.shane@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/1375/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>