FROMGIT: arm64: head.S: Initialise MPAM EL2 registers and disable traps

Add code to head.S's el2_setup to detect MPAM and disable any EL2 traps.
This register resets to an unknown value, setting it to the default
parititons/pmg before we enable the MMU is the best thing to do.

Kexec/kdump will depend on this if the previous kernel left the CPU
configured with a restrictive configuration.

If linux is booted at the highest implemented exception level el2_setup
will clear the enable bit, disabling MPAM.

This code can't be enabled until a subsequent patch adds the Kconfig
and cpufeature boiler plate.

Signed-off-by: James Morse <james.morse@arm.com>
[Cherry-picked from commit fa0ff38f06b3 at git://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git

]
[Changed CONFIG name and use IS_ENABLED()]
Bug: 221768437
Change-Id: Iaf435d2350be696147a509915a7c714143c2aa50
Signed-off-by: Valentin Schneider <Valentin.Schneider@arm.com>