sh: intc - rework higlander irq code for r7780mp and r7785rp

This patch reworks the highlander irq code for r7780mp and r7785rp.
The same strategy as for the new R2D code is used here - the board
specific interrupts are now starting from HL_FPGA_IRQ_BASE. The code
for r7780rp is not touched due to lack of hardware.

Tested with CF, AX88796 on r7780mp and r7785rp. The touch switch
interrupt has also been tested on r7780mp.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>