arm64: mpam: Restore the expected MPAM sysregs on cpuhp

When a CPU is brought online, only the MPAM1_EL1 sysreg is set. For
completeness restore the MPAM0_EL1 register too.

This isn't expected to cause a problem as cpuhp happens from the idle
thread, which can't return to user-space.

Signed-off-by: James Morse <james.morse@arm.com>