platform: thead/c910: Remove SBI_PLATFORM_HAS_PMP

T-head c910 is a generic FPGA platform so we cannot
define PMP configuration for it in OpenSBI because
PMP configuration tend to be SOC specific.

Signed-off-by: Liu Yibin <yibin_liu@c-sky.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>