Updates to the HPM for openhwgroup/cv32e40p#432

Created by: jm4rtin

Includes:

• Unprivileged read-only shadow registers for the HPM
• Removal of mcounteren from machine mode only (PULP_SECURE=0)
• Implementation of mcounteren for user mode (PULP_SECURE=1)

This is for #432 (closed) and #448 (closed). Related documentation change: openhwgroup/core-v-docs#168

Change-Id: Ia2641b5d9670c872e22b3b805a378f339cf1331d Signed-off-by: John Eric Martin John.Martin@emmicro-us.com