arm64: cputype: Add Cortex-X4 definitions

Add cputype definitions for Cortex-X4. These will be used for errata
detection in subsequent patches.

These values can be found in Table B-249 ("MIDR_EL1 bit descriptions")
in issue 0002-05 of the Cortex-X4 TRM, which can be found at:

  https://developer.arm.com/documentation/102484/0002/?lang=en

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20240508081400.235362-3-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>