lib: sbi: Don't clear mseccfg.MML bit in sbi_hart_smepmp_configure()

The mseccfg.MML bit is a sticky bit which remains unchanged once set
so no need to clear it in sbi_hart_smepmp_configure().

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>