arch: riscv: remove unneeded context switch to gp register

RISC-V global pointer (GP) register is neither caller nor callee
register, and it's a constant value in the single ELF file. Thus, we
don't need to save/restore GP at ISR enter/exit. Remove it to optimize
context switch performance.

Signed-off-by: Jim Shu <cwshu@andestech.com>

include/arch/riscv/exp.h

@@ -51,7 +51,6 @@ struct soc_esf {
 
 struct __esf {
 	ulong_t ra;		/* return address */
-	ulong_t gp;		/* global pointer */
 	ulong_t tp;		/* thread pointer */
 
 	ulong_t t0;		/* Caller-saved temporary register */