arm64: proton-pack: Add new CPUs 'k' values for branch mitigation

Update the list of 'k' values for the branch mitigation from arm's
website.

Add the values for Cortex-X1C. The MIDR_EL1 value can be found here:
https://developer.arm.com/documentation/101968/0002/Register-descriptions/AArch>

Link: https://developer.arm.com/documentation/110280/2-0/?lang=en


Signed-off-by: James Morse <james.morse@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>