armv8: Fix get_sctlr() return type

armv8: Fix get_sctlr() return type

SCTLR_EL2 is a 64-bit register [1]. Return its value as long (64 bit)
instead of int (32 bit) in get_sctlr() to make sure it's not trimmed.

[1] https://developer.arm.com/documentation/ddi0595/2021-06/AArch64-Registers/SCTLR-EL2--System-Control-Register--EL2-?lang=en

Fixes: 0ae7653128c8 ("arm64: core support")
Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>