imx8mp_evk: Increase CONFIG_SYS_MALLOC_F_LEN

When booting imx8mp-evk the following allocation error
message is seen:

U-Boot 2021.01-rc3-00200-ge668bec96a5f (Dec 21 2020 - 14:36:42 -0300)

alloc space exhausted

Fix it by increasing CONFIG_SYS_MALLOC_F_LEN to 0x10000 like it
is done on other i.MX8MM/8MN boards.

Reported-by: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Andrey Zhizhikin <andrey.zhizhikin@leica-geosystems.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>