riscv: Do not build reset.c if SYSRESET is on

SYSRESET uclass driver already provides all the reset APIs, hence
exclude our own ad-hoc reset.c implementation.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Sagar Kadam <sagar.kadam@sifive.com>
Reviewed-by: Pragnesh Patel <pragnesh.patel@sifive.com>