arm64: Fix the incompatible pointer type build error for user_cache_maint_handler()

Refer to commit 8d56e5c5("arm64: Treat ESR_ELx as a 64-bit register"),
modify the parameter esr to unsigned long, otherwise it will
be detected by the compiler as an error.

Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>