riscv: restore global data pointer in trap handler

The gp register is used to store U-Boot's global data pointer. We should
not assume that an UEFI application leaves the gp register unchanged as
the UEFI specifications does not define who is the owner of the gp and tp
registers.

So the following sequence should be followed in the trap handler:

* save the caller's gp register
* restore the global data pointer
* serve interrupts or print crash dump and reset
* restore the caller's gp register

Cc: Abner Chang <abner.chang@hpe.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Rick Chen <rick@andestech.com>