Support Linux kernel v5.4
To support rpi4 kernel based on Linux kernel v5.4, add wrapper for
changed function name.
With the Linux mainline commit
9285ec4c8b61 ("timekeeping: Use
proper clock specifier names in functions") in v5.3, function
ktime_get_boot_ns() is changed to ktime_get_boottime_ns().
Change-Id: I80e00592adea9e888daaef035f2332e5e7528eef
Ref: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=
9285ec4c8b61d4930a575081abeba2cd4f449a74
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>