arm64: cpuidle: add asm/proc-fns.h inclusion

ARM64 CPUidle driver requires the cpu_do_idle function so that it can
be used to enter the shallowest idle state, and it is declared in
asm/proc-fns.h.

The current ARM64 CPUidle driver does not include asm/proc-fns.h
explicitly and it has so far relied on implicit inclusion from other
header files.

Owing to some header dependencies reshuffling this currently triggers
build failures when CONFIG_ARM64_64K_PAGES=y:

drivers/cpuidle/cpuidle-arm64.c: In function "arm64_enter_idle_state"
drivers/cpuidle/cpuidle-arm64.c:42:3: error: implicit declaration of
function "cpu_do_idle" [-Werror=implicit-function-declaration]
   cpu_do_idle();
   ^

This patch adds the explicit inclusion of the asm/proc-fns.h header file
in the arm64 asm/cpuidle.h header file, so that the build breakage is fixed
and the required header inclusion is added to the appropriate arch back-end
CPUidle header, already included by the CPUidle arm64 driver, where
CPUidle arch related function declarations belong.

Reported-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>