spl: increase SPL_SYS_MALLOC_SIZE when using BIOSEMU on RISC-V

spl: increase SPL_SYS_MALLOC_SIZE when using BIOSEMU on RISC-V

If BIOSEMU is compiled for RISC-V (SiFive Unmatched board) and the function
dm_pci_run_vga_bios() is executed, U-Boot stops with error message saying
that the SPL malloc pool is too small. So increase the default pool size
when both BIOSEMU and RISCV parameters are set.

Signed-off-by: Yuri Zaporozhets <yuriz@qrv-systems.net>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>