arm64: el2_setup.h: fix spelling typo in comments

- "evailable" -> "available"

Signed-off-by: Prathu Baronia <prathubaronia2011@gmail.com>
Reviewed-by: Mukesh Ojha <quic_mojha@quicinc.com>
Link: https://lore.kernel.org/r/20230123110639.10473-1-prathubaronia2011@gmail.com


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>