powerpc/rtas: Replaced simple_strtoull() with kstrtoull()

The simple_strtoull() function is deprecated in some situation, since
it does not check for the range overflow, use kstrtoull() instead.

Signed-off-by: Chen Huang <chenhuang5@huawei.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210526092020.554341-1-chenhuang5@huawei.com