include: sbi: fix CSR define of mseccfg

Because the CSR names in the spec are mseccfg and mseccfgh. Remove
CSR_MSECCFG_LOWER and CSR_MSECCFG_UPPER and directly define
CSR_MSECCFG and CSR_MSECCFGH.

Signed-off-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>