hw/misc: Add Microchip PolarFire SoC SYSREG module support

hw/misc: Add Microchip PolarFire SoC SYSREG module support

This creates a minimum model for Microchip PolarFire SoC SYSREG
module. It only implements the ENVM_CR register to tell guest
software that eNVM is running at the configured divider rate.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 1603863010-15807-7-git-send-email-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>