arm64: Stop trashing the padding after boot_el

[andrew]

Summary

When building struct arm64_bootparams to pass to initarm we store the
boot_el field as a 64-bit value, however it is defined as an int which
is 32-bits.

Switch to store using the 'w' register as this will store a correctly
sized value.

Previously this would trash the trailing padding, so is only a
correctness issue.

Sponsored by: Arm Ltd