sh-pfc: r8a7790: Don't use GPIO enum entries

Refactor the GPIO macro magic to use GPIO numbers directly instead of
the GPIO_GP_x_y enum entries. This will allow removing the GPIO enum
entries from the mach/r8a7790.h header.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>