riscv: alloc space exhausted

When trying to run qemu-riscv64_smode_defconfig with 32 harts booting
fails. The debug UART shows a message

    alloc space exhausted

32 is the current maximum number of harts for machine virt in QEMU 7.0.

Raise the default for SYS_MALLOC_F_LEN to 16 KiB.

Move the setting to /Kconfig where we define SYS_MALLOC_F_LEN for
other architectures too.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Rick Chen <rick@andestech.com>