riscv: also select ARCH_KEEP_MEMBLOCK if kexec is selected

riscv: also select ARCH_KEEP_MEMBLOCK if kexec is selected

On RISC-V, also select ARCH_KEEP_MEMBLOCK if kexec is selected, not
only if ACPI is selected.  This is because kexec requires the memblock
areas to be kept after boot to initialize the secondary kernel.  This
is needed for both Device Tree and ACPI platforms.

Signed-off-by: Han Gao <gaohan@iscas.ac.cn>
Link: https://patch.msgid.link/20260519165546.123105-1-gaohan@iscas.ac.cn
[pjw@kernel.org: change to add the dependency on kexec, rather than making it unconditional;
 rewrite the patch description accordingly]
Signed-off-by: Paul Walmsley <pjw@kernel.org>