RISC-V: Add interrupt related SCAUSE defines in asm/csr.h

This patch adds SCAUSE interrupt flag and SCAUSE interrupt related
defines to asm/csr.h. We also use these defines in kernel/irq.c and
express SIE/SIP flags in-terms of SCAUSE interrupt causes.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>