riscv: cpu: correctly handle the setting of CPU_FEAT_MMU bit

The conditional check to read "mmu-type" from the device tree
is not rightly handled due to which the cpu feature doesn't include
CPU_FEAT_MMU even if it's corresponding entry is present in the device
tree.

The initialization of cpu features is now taken care in cpu-uclass
driver, so no need to zero out cpu_freq in riscv_cpu driver and can be
removed.

Signed-off-by: Sagar Shrikant Kadam <sagar.kadam@sifive.com>
Reviewed-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>